Reliable downhole data transmission system

ABSTRACT

A downhole signal transmission system provides electric radiofrequency signals that are coupled to electrically conductive or non-conductive fluids through electrical insulators. A plurality of signal repeaters are tuned to the frequencies of the radiofrequency signals, and a plurality of transmission lines terminated by resonance circuits are also provided such that the terminating resonance circuits resonate on the frequencies of the electric radiofrequency signals. The plurality of signal repeaters and plurality of transmission elements are arranged to be redundant such that a failure of one or more of the signal repeaters or a failure of one or more of the transmission elements does not substantially affect the operation of the data transmission system. The signal repeaters and transmission elements also are arranged such that a failure of any of the signal repeaters or a failure of any of the transmission elements is communicated to the surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/546,059 filed Jul. 11, 2012, now U.S. Pat. No. 8,704,667issued Apr. 22, 2014, which is a continuation of U.S. patent applicationSer. No. 12/470,842, filed May 22, 2009, now U.S. Pat. No. 8,242,928issued Aug. 14, 2012, which, in turn, claims priority under 35 U.S.C.§119(e) to U.S. Provisional Patent Application Nos. 61/128,582, filedMay 23, 2008, and 61/206,550, filed Feb. 2, 2009. The contents of theseapplications are hereby incorporated by reference in their entireties.

TECHNICAL FIELD

The present invention relates to the field of data transmission systems,in particular to data transmission systems suitable for downhole use,such as on a drill string used in oil and gas exploration, on completionstrings, or on casing strings. The present invention is especiallyuseful for obtaining downhole data or measurements while drilling aswell as sending commands from the surface to downhole drilling equipmentor sensing instrumentation.

BACKGROUND

There are a number of textbooks available that describe the processesinvolved in drilling for oil and gas. Examples of such textbooks are“Petroleum Well Construction” by Economides, Watters and Dunn-Norman,John Wiley & Sons, West Sussex, UK, 1998; “Applied Drilling Engineering”by Bourgoyne, Jr., Chenevert, Millhelm and Young, Jr., SPE TextbookSeries, Vol. 2, Society of Petroleum Engineers, Richardson, Tex., 1991;or “Drilling Technology—In Nontechnical Language” by S. Devereux,PennWell Corp., Tulsa, Okla., 1999. Reference may be made to thesetextbooks for an understanding of general drilling processes.

A drilling operation suitable for implementing the present invention isshown in FIG. 1. The drill rig 10 drives a drill string 20, which iscomposed of a large number of interconnected sections 30, called pipejoints. The bottom of the drill string is composed of heavy-weight pipesections 40, called drill collars. In a typical drilling operation, therig rotates the drill string and thus the bottom hole assembly (BHA) 50.The BHA 50 may contain various instrumentation packages, possibly a mudmotor or a rotary-steerable tool, stabilizers, centralizers, drillcollars and the drill bit 60. The drill string and all downholecomponents are hollow, allowing for drilling fluids to be pumped fromthe surface to the bit, with the drilling fluid returning to the surfacein the outer annulus between the drill string and the formation forcleaning and re-circulation. The drill string 20 may contain additionalsections of heavy-weight drill pipe and/or specialized equipment such asdrilling jars.

The two most common drive systems are the rotary-table system and thetop-drive system. The rotary-table system, shown in FIG. 1, engages thedrill string through the kelly bushing 65 and the kelly 70, causing thedrill string 20 to rotate while the kelly 70 is free to move up and downas the pipe is lowered into the ground or is lifted from the borehole.As the borehole deepens, pipe joints 30 are periodically added to thetop of the drill string 20 by means of rotary shoulder connections thatprovide mechanical strength and hydraulic seals. A top-drive system doesnot require a kelly 70; instead, the entire drive mechanism moves up anddown with the top end of the drill string 20. A top-drive systemfacilitates and accelerates the drilling process; however, it is alsomore expensive than a rotary-table system.

FIG. 2 shows a commonly used pipe joint 30 comprising a “box” tool joint301 at the top, a long tubular pipe body 302 and a “pin” tool joint 303at the bottom. A typical length for a pipe joint is 31 ft. Both pin 303and box 301 are equipped with conical threads that, when joined, form arotary connection. The two primary purposes of the connection are thetransmission of mechanical forces such as torque, tension andcompression between pipe joints 30 and to provide a liquid-tightmetal-to-metal seal at the outer interface. The conical thread can betypically made-up by hand and is tightened using pipe tongs or motorizedspinners, a process that puts the pin 303 under tension, the box 301under compression and the metal seal interfaces 305 also undercompression. This compressional load must exceed the tensional loads theseal experiences during bending and flexing in the hole to keep themetal-to-metal seal intact. The interior walls of the pipe joint 30 maybe coated with a high-performance phenol-based epoxy compound. Thiscompound is a high-quality dielectric insulator that inhibits corrosionof the metallic pipe and reduces friction losses in the fluid. Thethickness of the applied dielectric film is about 10-12 mils (0.25-0.3mm). Commercially available examples of such compounds are “TK-236” or“TK-34”, both available from Tuboscope, Houston, U.S.A.

The downhole instrumentation packages collect information about thedrilling process, about the formations being drilled, and about thefluids contained in those formations. In current practice, most of thisdata is stored in memory and later retrieved after the instrumentationhas been brought back to the surface. A very small and compressed amountof information, however, is typically sent in real time to the surfaceusing one of the currently available telemetry systems. The amount ofinformation available in real time on a typical drilling telemetrysystem used to be adequate in the early times of directional drilling. Afew bits of information describing the bit orientation with respect tothe earth's gravitational and magnetic fields are already useful forfollowing a pre-defined well path. Today, however, commercially viablereservoirs tend to be much more complex than those exploited in the pastand the recovery rates of the oil in place must be constantly increasedto make the remaining hydrocarbon reservoirs last longer. This alsomeans that well trajectories cannot be fully pre-defined based onseismic data or data from offset wells. Instead, well trajectories aremore and more determined while a hole is being drilled and arefine-tuned literally on a foot-by-foot basis. To accomplish this task, alot more formation evaluation data must be brought to the surface andmust be studied and interpreted while drilling is progressing. Theinterpretation results may or may not require adjustments to the welltrajectory, which are communicated back to the rig site. The rigequipment in turn communicates these adjustments to the downholeequipment. An example for a downhole imaging device that generates largeamounts of formation evaluation data while a hole is being drilled isdescribed in “Field Testing of an Advanced LWD Imaging ResistivityTool,” by Prammer et al., SPWLA 48th Annual Logging Symposium, Austin,Tex., 2007. Since the drilling process is relatively slow and formationdata can be compressed by the downhole electronics, a transmission ratealong the drill string of about 100-1,000 bits/second (bps) is required.In addition, the command channel from the surface to the downholeinstrumentation and the drilling system requires a transmission rate ofapproximately 1-100 bps.

Thus, it is a goal of the present invention to provide a downhole datatransmission system that can uplink data from downhole to the surface ata rate of at least 100 bits/sec, but preferably also at rates of1,000-100,000 bps, and that can downlink data from the surface at a rateof at least 10 bps, but preferably at a rate of 10-1,000 bps.

The need to transmit data from a downhole location reliably has beenrecognized for a very long time. See, for example, U.S. Pat. No.2,000,716, granted to Polk in 1935. Since then, a plethora ofcommunications systems have been proposed and implemented with varyingsuccess. The obvious approach, running a continuous electrical oroptical cable between the downhole location and the surface, runs intooperational problems because every time a pipe joint is added or removedfrom the drill string, the entire cable must be lifted from the string.To address this problem, Exxon Production Research Company developed acable system where the cable is stored downhole and is paid out asneeded. See also: Robinson, L. H.: “Exxon Completes Wireline DrillingData Telemetry System,” Oil & Gas Journal, Apr. 14, 1980, pp. 137-148.However, the operational complications using long cables lead to asegmented-cable approach. The 1935 Polk patent falls into that category.Polk uses sections of an electrically insulating liner “ . . . which maybe made of synthetic resin, varnished cambric, asphaltum or othersuitable material.” Inside the insulating liner, Polk places metaltubes, which are electrically connected to each other by metal springs.Using the metallic drill pipe as the return conductor, an electricalcircuit can be established and d.c. or a.c. signal currents can flow upand down the drill pipe. Liners, however, have a range of technical andeconomical shortcomings. First, a typical 15,000-ft. drill string wouldrequire the installation of 500 30-ft. sections of liner, which woulddramatically increase the capital expense for the drill string. Second,a feature like a thin-walled tube protruding from the box section of therotary joint is not compatible with normal rig operations, in whichheavy downhole components can swing laterally against each other duringmake-up and break-out, crushing any fragile elements that protrude andinterfere with that motion.

Shell Development Company developed an electrical telemetry system basedon modified pipe joints with electrical contact rings in the matingsurfaces of each tool joint. A wire inside the pipe bore connects thosecontacts on each end. See Denison, E. B.: “High Data Rate DrillingTelemetry System”, Journal of Petroleum Technology, February 1979, pp.155-163. Again, operational problems exist with these kinds of systems,as they tend to short out unless each connection is carefully cleanedand prepared with special, non-conductive “pipe dope”. U.S. Pat. No.6,123,561 to Turner et al. or U.S. Pat. No. 7,156,676 to Reynolds alsodescribe transmission systems that employ electrical contacts betweenpipe segments. However, these systems have in common that in order tocommunicate along the drill string many hundreds of individual elementswould have to be connected in series and all must function in order forthe entire system to be operational. Often, failure mechanisms in suchelectromechanical systems are intermittent and manifest themselves onlyunder the severe temperatures, pressures and mechanical stressesencountered downhole. Therefore, systems as described in or similar tothe '561 or '676 patents are usable only over short distances, e.g.between components of the BHA.

Thus, it is a goal of the present invention to provide a downhole datatransmission system that does not require special handling over andbeyond the normal care applied to rotary joints. In particular, thesystem should be compatible with the pipe dope compounds typically usedin the oil drilling environment, e.g. zinc-based compounds. The systemshould further be able to tolerate typical amounts of dirt, mud residueand other deposits that may or may not be electrically conductive in andaround the rotary connection and along the length of the tubular. Thesystem should also be able to tolerate partial short circuits againstground that are inevitable if the drilling fluid is conductive.

Frustration with the reliability of downhole electrical connections ledto the search for connector-less systems. U.S. Pat. No. 2,379,800,granted to Hare in 1945, describes transformer coupling at each pipejoint as well as an embodiment using telescoping condensers to providecapacitive coupling between pipe sections. U.S. Pat. No. 4,215,426 toLord added an amplifier and a battery in each pipe joint to thetransformer-based system. U.S. Pat. No. 4,605,268 to Meador, furtherrefines the idea of transformer coupling by specifying small toroidalcoils to transmit data across a rotary connection. U.S. Pat. No.4,788,544 to Howard uses an instrumentation package within the backboredbox section of a tool joint that is held captive by the pin nose of themated tool joint and transmits data across the rotary connection using amagnetic field and a Hall sensor. Similarly, U.S. Pat. No. 7,400,262 toChemali et al. uses an instrument package located in the backbored boxof the rotary connection. In this case, information between instrumentpackages is exchanged acoustically using the pipe body as transmissionmedium. It is easy to see that an acoustic transceiver rigidly heldcaptive between box and pin is inefficient in terms of convertingelectric into acoustic energy and vice versa. However, enough acousticenergy must be produced to clearly differentiate the signal against theacoustic noise background emanating from the bit and random locationsalong the drill string where contact with the wall is being made. Thus,any such device would consume large amounts of electric energy for verylittle transmission bandwidth, i.e. data throughput. The low bandwidthwould render such a system as no or little improvement over existingdata transmission systems, while the high power requirements would makeit uneconomical to power such a device from batteries and unfeasible topower such devices by energy harvesting techniques as described in theChemali patent. In addition, the large number of repeaters necessary,together with their electromechanical nature and the complete absence ofredundancy, would make it nearly impossible for the Chemali system tofunction over a useful time frame during downhole deployment.

Another data transmission system with transformer coupling is known as“IntelliPipe”. See, for example, U.S. Pat. No. 6,670,880 to Hall et al.,or Pixton, D.: “Very High-Speed Drill String CommunicationsNetwork—Report #41229R06”, Novatek Engineering, Provo, Utah, March 2003,or Pixton, D.: “Very High-Speed Drill String CommunicationsNetwork—Report #41229R14”, Novatek Engineering, Provo, Utah, June 2005.This system can be used only with a particular type of pipe connectors,known as double-shouldered tool joints. When pipe segments are joined,corresponding magnetic couplers embedded in the inner, secondary rotaryconnection shoulders make contact and form a closed magnetic circuit.Due to the necessity of special pipe and the difficult machininginvolved, such systems are very expensive. A typical “IntelliPipe” drillstring that is 15,000 ft. long may contain 500 coaxial cables, 1,000half-coupler elements, 1,000 connectors between magnetic couplers andcoaxial cable, approximately 10-15 signal repeater sub-systems, andvarious interfaces.

In the context of downhole data transmission, even the apparently simpleengineering problem of running an electric connection between the twoends of a pipe joint is surprisingly difficult. The “IntelliPipe” system(see the '880 Hall patent) uses a straight armored coaxial cable that isconstantly kept under tension and is only anchored at the tool joints.The U.S. Pat. No. 4,788,544 Howard patent proposes a coiled cable. Thesesolutions interfere with such basic oil field tasks as cleaning the boreof pipe joints. Drilling fluids can be aggressive chemicals, e.g. due totheir high salinity, which can damage the pipe by corrosion. Any kind ofcrevice or discontinuity in the flow stream attracts clay and/or othersolid deposits, which must be removed by wire brushes run inside thepipe. Left unattended, dirty pipe is easily pitted and corroded, whichcan lead to a fatal failure under load in the future. While drilling,fluid throughputs of 1,000 gal./min. are not uncommon, pumped through abore of a few inches in diameter. These fluids typically carry 1%-20%solid contents that, due to their high speed, carry massive abrasiveforces acting on every obstacle present in the flow cross section. Evenhigh quality steel or alloyed materials can be quickly eroded in thisenvironment.

Thus, it is a further goal of the present invention to provide adownhole data transmission system that does not interfere with the flowof drilling fluids; that maintains a smooth interior bore compatiblewith smooth, laminar flow; that is compatible with standard cleaningoperations, and that keeps the mechanical integrity of the pipe jointintact.

A very different category of telemetry systems establishes apoint-to-point connection between the downhole instrumentation and thesurface using the pipe string, the drilling fluid column or the earth astransmission medium. Common to these systems is the very high signalattenuation between transmitter and receiver and consequently very lowdata rates, typically in the range 0.3-30 bps. A particular family ofsystems in this category uses pressure pulses that travel inside thepipe string through the drilling fluid (“mud”). See, for example, U.S.Pat. No. 3,713,089 to Clacomb. Teleco Oilfield Company developed thefirst commercially successful mud-pulse system. Also see: Seaton, P. etal.: “New MWD-Gamma System Finds Many Field Applications”, Oil & GasJournal, Feb. 21, 1983, pp. 80-84. The achievable data rate ofpressure-pulse systems under realistic conditions is about 15 bps andrapidly falls with long drill strings and/or compressible fluids such asoil-based muds (OBM).

Another system in the point-to-point category transmits an extremelylow-frequency electromagnetic signal of a few Hertz from a downholelocation to the surface. An example of such a system is described inU.S. Pat. No. 4,087,781 to Grossi et al. The data to be communicated ismodulated onto the carrier signal. Problems with these systems are thevery low data rate, the one-directionality of the transmission andfailure to communicate near metallic casing and/or near certain earthformations such as salt domes.

A third type of point-to-point systems transmits mechanical signals suchas torque pulses through the body of the drill pipe. An example can befound in U.S. Pat. No. 3,805,606 to Stelzer et al. Of the aforementionedsystems, different implementations of pressure pulse communications arecurrently in commercial use as well as variants of extremelylow-frequency electromagnetic telemetry. However, it is believed that nosystem using mechanical or acoustic signaling along the pipe string isin commercial operation at this time.

To summarize, the apparent simple problem of establishing a signal pathalong a drill string has been found to be a very difficult engineeringproblem. Of all systems that divide the communications path intosegments corresponding to pipe joints, only “IntelliPipe” is in limitedcommercial use. Of all point-to-point transmission systems, only mudpulsing and extremely low-frequency electromagnetic communications havebeen fully developed. In principle, the segmented approach is much moreappealing because, due to frequent signal amplification and restoration,much higher data rates can be achieved compared to the point-to-pointapproach. The segmented approach, however, suffers from high initial andcapital costs and operational problems, mostly the lack of reliability.

It is a further goal of the present invention to enable reliablehigh-speed data transmission on existing pipe string hardware. There aremillions of feet (meters) of drill pipe in operation worldwide. Systemslike “IntelliPipe” require new pipe to be machined and instrumented toexacting specifications and cannot be retrofitted onto existing drillpipe of various provenance. A goal of the invention is adaptability toused drill pipe using only machining and coating operations that areavailable in many pipe-reconditioning shops worldwide.

It is another goal of the present invention to allow used pipe to bere-conditioned for use with the invention. Drill pipe is kept inoperation for many years by periodically cleaning and reconditioning allsurfaces and by re-cutting worn out threads.

It is perhaps an underappreciated fact that any segmented system willfail, no matter how reliable the individual component may be, givenenough of such components connected in series. This fundamental problemcan be better understood numerically. Consider an n-element datatransmission system, where each element functions fault-free with aprobability p throughout a single deployment interval (e.g., betweengoing into and pulling out of a hole). FIG. 3 depicts schematically sucha serial system. Assuming that the individual probabilities (lower-case)p are independent of each other, the probability (upper-case) P of theentire system to function without failure is:P(n,p)=p ^(n).

This function is plotted in FIG. 4 for n from 1 to 10,000 forprobabilities p of 0.99, 0.999 and 0.9999. The vertical line shows thetypical case of n=1000. Clearly, even given the rather unrealisticreliability of p=0.9999 (1 failure in 10,000 deployments) for a singleelement, the entire signal chain will break down approximately once inevery 10 deployments. Given that a downhole failure is likely to be alost-time failure (LTF), a 10% chance of failure is considered poorreliability.

One goal of the present invention is a data transmission system thatdoes not fail if a single element in a many-element configuration failsor if multiple elements at different locations fail during operation.Such functionality can be achieved by providing multiple datatransmission elements that under normal conditions operate in parallelas shown in FIG. 5. In case of a failure of an element, a parallelelement takes over the workload of the failed elements by increasing itsown workload. In mathematical notation, m elements are connected inparallel for a total of n*m elements per system. If the failureprobabilities 1−p of all elements are equal and uncorrelated, theprobability for the system to function without interruption is given by:P(m,n,p)=[1−(1−p)m] ^(n)*^(m).

This function is shown in FIG. 6 for the singly-redundant case of m=2.Again, n ranges from 1 to 10,000 and p is 0.99, 0.999 and 0.9999. Such asystem can be characterized as very reliable except for the poorestelement reliability of 0.99. The next step to a doubly-redundant systemwith m=3 is shown in FIG. 7 and the corresponding probabilities areplotted in FIG. 8. As can be seen, such a system is in fact morereliable than its parts and other failure mechanisms, such as failuresin the interfaces to the system, will determine the overall systemreliability.

The implementation of the “crossovers” (see FIGS. 5 and 7), which areessential for the isolation of a failure from the rest of the system,requires careful consideration. On one hand, “tight” coupling betweenelements is desired to avoid signal losses as much as possible. Tightcoupling schemes are electrical contacts, followed by magnetic coupling,such as used in “IntelliPipe”. On the other hand, the coupling should beas weak as possible so that a failed system does not interfere with thefunctionality of the remaining system. These considerations lead to theconclusion that, in a rather counterintuitive way, “weak” couplingschemes can provide the path to high system redundancy and reliability.

It is thus a further goal of the present invention to achieve a highlevel of system reliability by implementing a serial/paralleltransmission scheme in which single-point failures can be isolated andbypassed without deterioration in overall system functionality.

It is yet another goal of the present invention that the system candetect, diagnose and report all failures or problems as soon as theyoccur. An operator may choose to replace the affected drill stringsegment at the next opportunity. Such an opportunity may exist when thedrill string must be removed from the hole, due to, for example, aworn-out drill bit.

It is still another goal of the present invention to provide thenecessary information in an easy-to-understand format that does notrequire specialized knowledge about the inner workings of the system.Since the present invention employs a large number of instrumented datarepeaters, it is straightforward to include diagnostic and recoverycapabilities that are distributed throughout the system. Thesecapabilities include the capability to sense a local hardware failureand to report that failure to the surface. Such a report contains theserial number of the affected pipe segment, which simplifies thereplacement of that segment with one that is known to be working. Ahardware failure may be sensed by a drop in received signal strength ortransmission silence during a time interval in which a signal is to beexpected. The recovery capabilities include the capability to take overthe workload of a failed element by a working element that increases itsduty cycle to compensate for the increased workload.

Another goal of the present invention is a data transmission system thatcan be adapted to a variety of drill pipes, drill pipe parts and variousdownhole and surface equipment. This should include transmission betweencomponents or parts of components that move relative to each other suchas axial translation or rotation.

Yet another goal of the present invention is the provisioning of fullbidirectional data transfer between terminals at the surface anddownhole.

Still another goal of the present invention is the provisioning of datatransmission in both directions that appears error-free between endpoints, such that “soft”, i.e. transient, errors are detected andcorrected within the system.

It is a further goal of the present invention to provision aflexible-cost data transmission system. In certain applications, such asdrilling through well-known formations, only a modest amount of data isrequired. In other applications, such as drilling an exploratory well orsteering a steerable drill bit through a complicated reservoir zone, ahigh amount of data is required. Another example of a highly variableuser dataflow is seismic-while-drilling (SWD). In SWD, seismicmicrophones and the associated electronics are part of the downholeinstrumentation package. At times when drilling stops, such as when anew connection is made up, a seismic wave may be launched from thesurface, which is recorded downhole. These waveforms may be sent to thesurface in bursts and in real time using the data transmission system.Therefore, the data transmission system should preserve its resourcesduring times when only a small amount of data is relevant and shouldtransmit at high speed at times when large amounts of data are availableand are relevant for real time operations.

It has been a long-accepted limitation of while-drilling measurementsthat data can only be gathered in the BHA, where typically all sensorsare located. Thus, data along the borehole is only available shortlyafter a particular section has been drilled, while changes in thatsection that occur hours or days after the section has been drilled arenot detectable. Such changes may include the influx of formation fluidinto the borehole, a condition that depends among other factors onformation pressure, borehole pressure and the formation of mud cake onthe borehole wall. Such influx is typically detectable as localtemperature aberration because the borehole fluid and the formationfluids typically have different temperatures. It is also highlydesirable to measure borehole pressure along the entire wellbore inorder to equalize the pressure exerted by the formation towards theborehole, without applying too much pressure outwards, a situation thatcould ruin future production from a reservoir. Currently, the pressureand temperature profiles are simply estimated by linearly interpolationbetween surface values and BHA values, or worse, are calculated fromassumed gradients. The “IntelliPipe” system has introduced limitedcapabilities to deploy sensors along the borehole in the signalrepeaters. The “IntelliPipe” signal repeaters, however, are complexelectronics packages that are spaced out as far as possible 1,000-2,000ft. intervals along the drill string. Such course sampling isundesirable for gathering data along the entire wellbore.

Thus it is another goal of the current invention to enable datagathering along the wellbore with spacings between sensor points of aslittle as a single pipe joint, i.e. about 10 meters.

SUMMARY

The above-mentioned and other goals and advantages of the system willbecome apparent from the following detailed description of theinvention. As will become apparent from the following description, theinvention includes a downhole signal transmission system in whichelectrically conductive or non-conductive fluid comprises a portion ofthe signal path and in which electric radiofrequency signals are coupledto the fluid through electrical insulators. In an exemplary embodiment,the frequency of the radiofrequency signals is in the range ofapproximately 1 MHz to approximately 1 GHz. In the exemplary embodiment,a plurality of signal repeaters are tuned to the frequency of theradiofrequency signals. A plurality of transmission lines terminated byresonance circuits are also provided whereby the terminating resonancecircuits resonate on the frequency of the electric radiofrequencysignals.

The downhole data transmission system of the invention includes aplurality of signal repeaters and a plurality of transmission elementsarranged to be redundant such that a failure of one or more of thesignal repeaters or a failure of one or more of the transmissionelements does not substantially affect the operation of the datatransmission system. Preferably, the invention includes the feature thata failure of any of the signal repeaters or a failure of any of thetransmission elements is communicated to the surface. During operation,the signal repeaters receive and transmit radiofrequency signals and thetransmission elements transport radiofrequency signals between therepeaters. To provide the desired redundancy, the number of signalrepeaters used is substantially larger than the number of signalrepeaters necessary to receive and transmit data, and the number oftransmission elements is substantially larger than the number oftransmission elements necessary to transport data.

The downhole pipe section in which the downhole data transmission systemof the invention is implemented includes rotary connections and atubular pipe section arranged to transport radiofrequency signals overtransmission elements wherein the inner cross section of the pipesection is approximately circular and the transmission elements take upapproximately less than 5% of the cross sectional area around theperiphery of the pipe. Preferably, the pipe section is designed suchthat its mechanical strength is approximately equal to and, in any case,at least 95% of the mechanical strength of the pipe section without thetransmission elements. In an exemplary implementation, the transmissionelements are contained in the interior coating of the downhole tube.

The scope of the invention also includes a method of transmitting datadownhole by transmitting electrical signals between electrodes throughelectrical insulators and an electrically conductive or an electricallynon-conductive fluid. In an exemplary embodiment, the frequency of theelectrical signals is in the range of approximately 1 MHz toapproximately 1 GHz. The electrical signals are received and transmittedby a plurality of signal repeaters and the electrical signals aretransported by a plurality of transmission elements such that theelectrical signals cause electrical resonances in some of thetransmission elements.

In an exemplary embodiment of the downhole data transmission method ofthe invention, the method includes weakly coupling redundant elements ina downhole data transmission system such that a failure in one or moreof the redundant elements does not catastrophically affect the operationof the downhole data transmission system. In an exemplary embodiment,the approximate location of the failed element or locations of failedelements is communicated to the surface.

The methods of the invention also include a method of installing aplurality of transmission elements in a downhole pipe section whereinthe inner cross section of the pipe section is approximately circularand the transmission elements take up approximately less than 5% of thecross sectional area. Also, the mechanical strength of the pipe sectionis approximately equal to or, in any event, at least 95% of themechanical strength of the pipe section without the transmissionelements. The transmission elements may be contained in the interiorcoating of the downhole pipe section. Also, the transmission elementsmay be contained in grooves within the wall of the inner bore of thepipe section.

The invention also includes a coupling element and associated method oftransmitting radiofrequency signals in a downhole signal transmissionsystem comprising a stationary electrode and a movable electrode whereinelectric radiofrequency signals in the range of approximately 1 MHz toapproximately 1 GHz. are coupled between the electrodes throughelectrical insulators and a fluid. In an exemplary embodiment, thecoupling elements are part of a drilling jar or are part of a surfacecommunications sub.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a conventional drillingenvironment showing various downhole components.

FIG. 2 is a perspective view of a conventional pipe joint with rotaryconnections.

FIG. 3 is a conceptual illustration of a chain of transmission elementsconnected in series.

FIG. 4 is a plot of the system reliability function P=p^(n) for a chainof n transmission elements connected in series according to FIG. 3 withindividual element reliabilities of p=0.99, p=0.999 and p=0.9999.

FIG. 5 is an illustration of a chain of transmission elements connectedin parallel and in series for the case of two transmission paths inparallel with periodic crossovers between elements.

FIG. 6 is a plot of the system reliability functionP=(1−(1−p)^(m))^(n)*^(m) for a system according to FIG. 5 where thereare in total 2n elements with individual element reliabilities ofp=0.99, p=0.999 and p=0.9999.

FIG. 7 is an illustration of a chain of transmission elements connectedin parallel and in series for the case of three transmission paths inparallel with periodic crossovers between elements.

FIG. 8 is a plot of the system reliability functionP=(1−(1−p)^(m))^(n)*^(m) for a system according to FIG. 7 where thereare in total 3n elements with individual element reliabilities ofp=0.99, p=0.999 and p=0.9999.

FIG. 9 is a cut-open perspective view of a pipe joint prepared foraccepting data transmission elements and fitted with interior pipecoating suitable for data transmission in accordance with the invention.

FIG. 10 is an “x-ray” perspective view of a pipe joint showing variousdata transmission elements buried under the interior pipe coating.

FIG. 11 a is a conceptual, flattened layout of the buried electrodes ineach tool joint area and the transmission lines connecting them.

FIG. 11 b shows conceptually the cross sections A-A′, B-B′ and C-C′,whose locations are indicated in FIG. 11 a.

FIG. 12 is a conceptual diagram of a tool joint showing buriedelectrodes with capacitive and inductive circuit elements formingresonant circuits and the transmission lines coupling the resonantcircuits.

FIG. 13 a is a conceptual perspective view of the pin tool joint withtransmission elements installed where, for clarity, the buried electrodesystem has been drawn as if the coating is transparent.

FIG. 13 b is another perspective view of the pin tool joint showing thelocation of the embossed tool joint identification (ID) in the tongarea.

FIG. 14 a is a conceptual perspective view of the box tool joint withcertain transmission elements installed where, for clarity, the buriedelectrode system has been drawn as if the coating system is transparent.

FIG. 14 b is another perspective view of the box tool joint showing thelocation of the optional sensor insert.

FIG. 15 is a perspective view of the box tool joint with transmissionelements installed, including the repeater housing and the so-calledtaper ring.

FIG. 16 is a perspective view of the API-style repeater housing and theAPI-style taper ring.

FIG. 17 is an exploded view of the API-style repeater housing showingmechanical and electronic components of individual repeaters.

FIG. 18 is a conceptual diagram of a tool joint showing buriedelectrodes, transmission lines, repeaters, capacitive and inductivecircuit elements forming resonant circuits and the general areas whereelectrical signal and ground coupling between adjacent elements occurs.

FIG. 19 is a cross section through an API-type rotary connectionmodified to accept data transmission elements.

FIG. 20 is a cross section through a modified API-type rotary connectionin a partially made-up state with electrodes and transmission linesinstalled and coated with an epoxy system.

FIG. 21 a is a cross section through a modified API-type rotaryconnection with the data transmission system installed.

FIG. 21 b is a cross section through a modified API-type rotaryconnection with the data transmission system installed in a fullymade-up state.

FIG. 22 a is a cross section through a modified API-type rotaryconnection with the data transmission system installed and showing theapproximate locations and dimensions of sensor elements.

FIG. 22 b is a cross section through an API-type rotary connection withthe data transmission system installed and sensor elements installed.

FIG. 23 is an illustration of various interchangeable sensor insertswith different sensing functions.

FIG. 24 is a block diagram of a twin repeater using non-rechargeablebatteries.

FIG. 25 is a block diagram of a repeater core circuit.

FIG. 26 is a block diagram of a sensor insert circuit.

FIG. 27 is an x-ray perspective view of a made-up rotary connectionshowing transmission elements captured between the tool joint pin andthe back wall of the tool joint box.

FIG. 28 is a conceptual exploded see-through view of stacked pipe jointswith data transmission elements installed showing the cases of (a)straight transmission lines, and (b) twisted transmission lines.

FIG. 29 is a conceptual exploded see-through view of stacked pipe jointswith data transmission elements and sensor elements installed for thecases of (a) sensors installed as sensor inserts in pipe joints and (b)sensor installed in dedicated sensor subs.

FIG. 30 is a conceptual exploded see-through view of stacked surfaceelements of a kelly-type drill string with data transmission elementsinstalled.

FIG. 31 is a conceptual circuit block diagram of a surfacecommunications sub.

FIG. 32 is a conceptual exploded see-through view of stacked BHAelements with data transmission elements installed.

FIG. 33 is a perspective view of a downhole interface sub.

FIG. 34 is a conceptual circuit block diagram of a downhole interfacesub.

FIG. 35 is a conceptual exploded see-through view of an in-stringinstrument sub with stacked pipe joints above and below the instrumentsub.

FIG. 36 is a conceptual circuit block diagram of an instrument sub.

FIG. 37 (a)-(b) are conceptual see-through views of a drilling jar withdata transmission elements installed in the closed (a) and open (b)positions.

FIG. 38 is a conceptual perspective view of the movable and stationarycoupling electrodes within an instrumented drilling jar.

FIG. 39 is a conceptual cross section through the movable and stationarycoupling electrodes within an instrumented drilling jar.

FIG. 40 is a block diagram of a twin repeater using rechargeablebatteries.

FIG. 41 is a conceptual view of a set of repeaters in their storage,transport, charging, and testing container.

FIG. 42 is a conceptual exploded see-through view of surface testequipment and a string of pipe joints under test.

FIG. 43 is another conceptual exploded see-through view of surface testequipment and a single pipe joint under test.

FIG. 44 is another conceptual exploded see-through view of surface testequipment, a single pipe joint under test, and repeaters exchangingdata.

FIG. 45 is a conceptual circuit block diagram of a surface testequipment circuit.

FIG. 46 shows modulation of the radiofrequency by on/off keying (OOK)and Manchester modulation.

FIG. 47 (a)-(b) show an example of the formatting of data packets.

FIG. 48 shows an example of packetized messages of different lengthstransmitted in TDMA channels.

FIG. 49 shows the transmission of a data packet through the transmissionsystem at low data rates.

FIG. 50 shows the occurrence of a transient transmission error and itscorrection at low data rates.

FIG. 51 shows the transmission of data packets through the transmissionsystem at high data rates in unidirectional mode.

FIG. 52 shows the occurrence of a transient transmission error and itscorrection at high data rates.

FIG. 53 shows the transmission of data packets through the transmissionsystem at high data rates in bidirectional mode.

FIG. 54 shows a portion of the transmission system in the process ofadding another pipe joint.

FIG. 55 shows the transmission of data packets in “swarm” mode, in whichevery repeater reports status and data to the surface.

FIG. 56 depicts the series of events that occur when one or morerepeaters located in the same repeater housing fail.

FIG. 57 depicts the series of events that occur when a single repeatersheds transmission workload in order to increase the repeater's expectedservice lifetime.

FIG. 58 is a simplified flow diagram of a repeater's operation.

FIG. 59 is a diagram of an example wellbore layout.

FIG. 60 is a conceptual diagram showing distributed sensors picking upand processing seismic waves arising from acoustic drill bit noise.

FIG. 61 is a conceptual diagram showing distributed sensors picking upand processing seismic waves through the earth formation from a seismictransmitter at the surface.

FIG. 62( a)-(b) is a conceptual diagram showing distributed sensorspicking up seismic waves through the earth formation from acoustictransmitters located on in-string instrumentation subs.

FIG. 63( a)-(b) is a conceptual diagram showing distributed sensorspicking up seismic waves through the earth formation from acoustictransmitters located on in-string instrumentation subs in the same or inan adjacent well.

FIG. 64 is a screenshot from a network analyzer CRT showing theelectrical characteristics of a prototype electrode system.

FIG. 65 is a screenshot from a spectrum analyzer CRT showing theradiofrequency carrier signal received through a prototype of the datatransmission system in the cases of water and or air as drilling fluid.

FIG. 66 is a cross section through a double-shoulder rotary connectionmodified to accept data transmission elements.

FIG. 67 is a cross section through a modified double-shoulder rotaryconnection in a partially made-up state with electrodes and transmissionlines installed and coated with an epoxy pipe coating system

FIG. 68 is a cross section through a modified double-shoulder rotaryconnection with the data transmission system installed.

FIG. 69 is a cross section through a modified double-shoulder rotaryconnection with the data transmission system installed in a fullymade-up state.

FIG. 70 is a computed plot showing displacement current lines within adouble-shoulder rotary connection with the data transmission systeminstalled.

FIG. 71 is a perspective view of a repeater housing for adouble-shoulder rotary connection.

FIG. 72 is a perspective view of a passive insert for a double-shoulderrotary connection.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

A detailed description of illustrative embodiments of the presentinvention will now be described with reference to FIGS. 9-72. Althoughthis description provides a detailed example of possible implementationsof the present invention, it should be noted that these details areintended to be exemplary and in no way delimit the scope of theinvention.

The data transmission system described in the present invention inconnection with FIGS. 9-72 is intended for integration into existing ornew drill string components, bottom-hole assemblies (BHAs), or otherdownhole equipment to provide fast, bidirectional communication betweensurface computer equipment and downhole instrumentation. In addition,the data transmission system is intended to provide for versatiledata-gathering and data-processing capabilities along an entire drillstring or a downhole tubular for assessing drilling conditions and forevaluating the formation in the general vicinity of the wellbore.

The following discussion mostly focuses on the integration of the datatransmission system into the pipe joints, as these are the most numerouscomponents in a downhole component string. Other elements that may beconnected in series with the pipe joints from time to time and that alsomay contain data transmission elements are: pup joints, which are shortsections of pipe joints, drill collars and heavyweight pipe, jars,kellys, valve subs, saver subs, crossover subs, sensor subs, interfacesubs, communication subs, instrument subs, subs in general and checkoutequipment. Most of these components are deployed downhole, while others,such as the kelly, communication subs and checkout boxes remain on thesurface.

FIG. 9 shows a conventional pipe joint 30 of the type shown in FIG. 1 incut-open view with partially installed data transmission systemcomponents in accordance with the invention. The box end of the rotaryconnection has been back-bored according to “Specification for RotaryDrill Stem Elements—API Specification 7,” 40^(th) Edition, November2001, American Petroleum Institute. The resulting inner cavity 310 isintended to house an electronics package (the “repeater housing”) thatwill be described later. The pin 320 of the rotary connection has beentrimmed to the length dimension necessary to capture and hold firm suchan electronics package between the pin 320 and the back wall 325 of thebox when the rotary connection is fully tightened. The flow channels 330of high-end premium pipe joints are typically coated withhigh-performance epoxy-resin powder coat systems 400. Before the coats400 are applied, the pipe joint 30 is thermally cleansed of any residuesand sandblasted using aluminum-oxide powder. In addition to thesestandard procedures and in preparation of accepting the datatransmission system, the sandblasted joint has to be deburred bygrinding/removing residual aluminum-oxide particles and any protrudingsurface imperfections. Not visible in FIG. 9 is a system of electrodesand transmission lines that are buried inside the dielectriccorrosion-inhibiting coating system. The burial is achieved by firstcoating the sandblasted, deburred and smoothed bare-metal interiorsurface with a liquid epoxy primer such as “TK-8007” available throughTuboscope, Houston, then applying the electrode systems and finallyapplying a dielectric overcoat such as “TK-236”, also available throughTuboscope, Houston. Alternatively, a first coat of the overcoat may beapplied to the primer as raw powder. On this first coat, the electrodesare applied, over which a second coat of overcoat powder is applied.Typical thicknesses for the primer are 2 mils (˜0.05 mm) and for theovercoat 10 mils (˜0.25 mm). The primer and the electrodes are appliedat lower temperatures, while the overcoat powder is applied as drypowder to the heated pipe, where the powder melts into a gel and thencures at about 200° C. for about 30 minutes. During the curing process,the epoxy gel flows around, into and under the electrodes and, afterreaching its glass-transition temperature, hardens through cross-linkingto a solid material of glass-like hardness. During the powderapplication and during the curing process, the pipe joint is preferablyrotated around its long axis to create enough centrifugal force suchthat the gel flows freely inside the bore achieving uniform thicknesswithout accumulating in low spots due to gravity. Such rotation alsoapplies centrifugal forces to the embedded electrode systems causingthem to further sink into the soft gel while entrained gas bubbles arereleased through the gel's surface. After the curing process, theelectrode systems and the transmission lines are integral andinseparable components of the interior pipe coating system withouthaving electrical contact to other components. Therefore, thecorrosion-inhibiting properties of the coating system remain intactwhile the fragile electrode systems are fully protected from the harshdownhole environment.

FIG. 10 is the see-through view of the pipe joint 30 shown in FIG. 9.Visible are the buried ring-shaped box electrode system 350, the buriedtransmission lines 360, and the buried ring-shaped pin electrode system370. Each electrode system includes individual electrodes, which may bemanufactured from double-sided copper-cladded polyimide film. Such filmsare available under the trade name “Pyralux” from DuPont ElectronicMaterials, Research Triangle Park, N.C. 27709. Suitable DuPont productsare AP 8555 or AP 9151, which employ as dielectric a 5-mil (0.127 mm)thick polyimide sheet sandwiched between two copper layers. The copperlayers are preferably electrochemically deposited (suffix “-E” to theDuPont product number). Alternatively, sheets of polyimide may beobtained from DuPont and coated using well-known spattering techniqueswith thin metals layers such as copper, silver, gold, or aluminum. Thetransmission lines may be simply realized by using standard fine-gaugecoated magnet wire, such as AWG #36, for one conductor and utilizing thepipe's metal body for the return path. Each electrode system may bedivided into arbitrarily many individual electrodes, wherein eachelectrode in the box has a counterpart electrode in the pin. Suchprimary and counterpart electrodes are connected by a transmission line,i.e. by one or more wires, to each other. No portion of the electrodesystems or transmission lines is intended to make electrical contact tothe pipe body, although such an electrical contact would not necessarilyinterfere with the proper operation of the data transmission system.Instead, the operating frequency or frequencies are chosen to be highenough such that the capacitances between the electrodes and the pipebody are sufficient to complete an electric a.c. circuit between the boxelectrodes and the pin electrodes. It has been found that dividing eachelectrode ring into two or three electrodes is sufficient to achieve ahigh degree of redundancy and robustness. Preferably, a single wire isused to connect each pair of corresponding pin and box electrodes insuch a way that the wire may break or may short to ground, therebydisabling one pin/box pair, but without substantially affecting theremaining one or two pin/box electrode pairs.

FIG. 11 a is a flattened conceptual layout of an example electrodesystem. The patterns shown are preferably photolithographically etchedinto the top and bottom metal layers (“plates”) attached to thepolyimide sheet. Both line thickness and line separation are about 10mils (˜0.25 mm) After etching, the exposed polyimide is vaporized, e.g.by a focused high-power laser beam, leaving a finger-like electrodestructure as shown in cross-section A-A′, FIG. 11 b. Also shown in FIG.11 b is the shallow trench 375 created by deburring/honing the pipe andin which the electrode structure is deposited. The depth of thesetrenches 375 is only about 5 mils (˜0.13 mm) and preferably does notexceed 10 mils (˜0.25 mm). During the curing process, the epoxy gel 400is free to flow around, into and under the electrode structures 370,thus completely encapsulating them. Note that FIGS. 11( a)-(b) aresimplified by showing the metal surface as absolutely smooth. Inreality, the irregularities in the pipe's surface are of the same or oflarger magnitude than the thickness of the electrode system. Therefore,the individual “fingers” are not parallel but rather follow the pipemetal's surface contours. What matters only is that the distance betweenthe top and bottom metal layers, which is determined by the polyimidethickness, remains constant.

FIG. 11 a shows as an example two segments per electrode system: Eachsegment is a resonant circuit comprised of a capacitive-type portionintended to electrically couple to the environment and an inductive-typeportion with little capacitive coupling. The inductor is realized by alength of conductor 380 attached to the capacitor 385 on the upper plateand terminated through a through-hole (“via”) to the lower plate. Asshown, different capacitances 385 are compensated for by selectingdifferent lengths for the inductors 380. It has been found advantageousto tune the resonant tank circuit slightly lower than the intendedoperating frequency due to the inductive load presented by the attachedtransmission line. For an operating frequency of 27 MHz, the tuningfrequency would be around 26-26.5 MHz. Pairs of pin/box electrodesystems are connected by fine-gauge wires 360. A cross section B-B′ isgiven in FIG. 11 b. The wire 360 may be round or rectangular in crosssection and is fine enough to be completely encapsulated during thecoating process. Preferably, the wire is pre-coated with insulatingmaterial, although this is not an absolute requirement. The wires areattached to the electrode's upper plate through soldering or brazing ina wire channel formed by etching away the lower plate and vaporizing thepolyimide dielectric in this section (see FIG. 11 b, section cut C-C′).

FIG. 64 provides a screenshot from the CRT of a network analyzer HP3585Ashowing the electrical characteristics of a prototype electrode systemembedded in pipe coating. The horizontal axis is the frequency between 0and 40 MHz, the vertical axis represents the signal reflected from theelectrode in an impedance bridge. The vertical axis is logarithmicranging from −10 dBm at the top to −110 dBm at the bottom. The frequencymarker is set at 27 MHz where a flat “shoulder” with a width of about 2MHz indicates the proper parallel resonance condition where theelectrode system acts as a tank circuit with a Q of about 10. Thisfrequency region is suitable for data transmission. To the right of theoperating range exists a peak indicating a resonance caused by theelectrode system's capacitive behavior resonating with the inductance ofthe attached wire, i.e. the transmission line. To the left of theoperating range exists a deep valley indicating a series resonancecondition caused by the electrode's system inductive behavior resonatingwith the capacitance of the electrode to the pipe metal. The particularfrequencies of the two latter resonances are ill-defined; however, theyare guaranteed by design to occur outside the useful 2-MHz operatingwindow.

FIG. 12 further explains the electrical characteristics of the buriedelectrode systems. As explained before, the electrodes and transmissionlines are completely coated with a dielectric 400, leaving nointentional contact to the pipe body. For clarity, the electrodes arepictured as lumped-element L-C parallel resonant circuits 350 and 370.The dielectric material is important in providing displacement currentpaths both from the “upper” or “hot” plates to the environment and fromthe “lower” or “cold” plates to the pipe body. The connecting wires 360,the dielectric and the pipe metal form low-quality transmission lineswith relatively high characteristic impedances of around 80 ohms. Theexact parameters of the lines are not essential, since only weakcoupling between the resonant circuits in the box and in the pin isdesired. Sufficient coupling is achieved when electric resonance at oneend induces resonance at the other end, while avoiding excessivedetuning due to strong coupling. A parasitic resonance occurs when theelectrode resonates with capacitance of the lower plate against the pipemetal. This resonance strongly short-circuits all signals and occursbelow the tuning resonance of the electrodes. Therefore, the operatingfrequency at which the system achieves high impedance against the pipebody is at or above the electrode's self resonance frequency. The Q ofthis resonance is around 10, which is rather low, but desirable forseveral reasons. Operating at or around 27 MHz with a Q of 10 createsusable bandwidth of about 2 MHz, large enough to accommodate a 1-MHzwide signal plus frequency variations due to downhole conditions. Suchchanges, for example, are the decrease in electric permittivity of thepolyimide dielectric due to temperature, the increase in capacitance dueto compression under pressure and variable electric loading due todielectric and conductive properties of the drilling fluids.

FIG. 13 a is a conceptual, perspective view of the pin end 303 of arotary connection with data transmission elements installed. Forclarity, the installed electrode system 370 is shown as if the epoxycoating 400 is transparent. As can be seen, the pin electrode system 370is set back from the pin face 375 by approximately ¼″ (˜6 mm) to protectthe electrode 370 in case the coating becomes damaged near the pin face375. FIG. 13 b is a top view of the pin end showing the pin 320 and thepin thread and the general location within the tong area where a uniquepipe joint identification (ID) number or alphanumerical string 380 isembossed. This ID 380 is used to visually reference a particular rotaryconnection and is also stored electronically within the pipe joint andalso offline (see discussion below).

FIG. 14 a is a similar conceptual, perspective view of the box end 301.For clarity, the data transmission elements “repeater housing” (a.k.a.“twin repeaters”) and “taper ring” have been omitted (shown as notinstalled). The epoxy coating 400 is drawn as if it is transparent. Thebox electrode system 350 is mounted on the interior box wall in alocation determined by the dimensions of the repeater housing (furtherexplained below). FIG. 14 b is a top view of the box end 301 showing thegeneral location within the tong area where an optional sensor insert390 can be installed. The sensor insert 390 houses one or moreenvironmental sensors and communicates with the data transmission systeminside the pipe joint.

FIG. 15 is a perspective view of the box end 301 with all datatransmission elements installed. Partially visible is the “repeaterhousing” 410 (or “twin repeaters”) and the “taper ring” 420 below. Theseelements are tightly fitted into the backbored space within the box end301. The exposed face of the repeater housing is the contact ring 430,which will make mechanical and electrical contact with the pin face ofthe next pipe joint once the rotary connection is tightened. Most of therepeater housing's and the taper ring's surfaces are coated with epoxypipe coating 400, with the exception of the contact ring 430, whichremains bare metal because of the mechanical friction and desiredelectrical contact at this interface.

FIG. 16 is a perspective view of the repeater housing 410 and the taperring 420. The repeater housing 410 has the shape of a short length ofpipe, while the taper ring 420 conforms to the conical shape of the endof the box 301. The repeater housing mainly consists of a contact ring430 at the exposed end or two contact rings 430 at each end, separatedby elastomeric seals 440 (such as “Viton Extreme” fluoroelastomer, madeby DuPont, Wilmington, Del.) from the repeater pressure housing itself.The repeater housing 410 is a short section of steel tubing with typicaldimensions of O.D. 4.2 inches, I.D. 3 inches and length 1.75 inches. Theyield strength of the steel is preferably 120,000 psi or higher.

FIG. 17 shows the repeater housing 410 in exploded view. The repeaterpressure housing has revolver-style chambers 450 that hold theelectronic components of the repeaters. The chambers 450 are dimensionedto accommodate commercially available high-temperature lithiumbatteries. A suitable primary-cell lithium battery is part no. 4037,size ½ AAA lithium cell 10-25-150, available from Electrochem, Clarence,N.Y. The dimensions of this cell are 9.5 mm diameter and 25.4 mm (1inch) in length. At room temperature, this cell delivers a nominalcharge of 0.5 ampere-hours at 2 mA discharge current and has a voltageof about 3.5 V. The repeater electronics is housed in cartridges 460with dimensions identical to that of the Electrochem #4037 battery.Although one repeater housing can hold enough cartridges for as many asfour repeaters, in practice only 2-3 repeaters are used. For clarity,FIG. 17 only shows the four cartridges that typically make up a singlerepeater with the remainder of the chambers 450 shown as empty. In theexample shown, one cartridge is occupied by the battery, anothercartridge holds a capacitor for short-term energy storage and twocartridges hold the electronics. A ferrule 465 electrically connects therepeater electronics to the contact rings 430.

Repeaters can be powered by a single battery or by more than onebattery. Collar joint repeaters may be powered by two batteries inparallel, a configuration made necessary by the longer high-temperatureexposure of these repeaters, resulting in increased self-discharge ratesof the lithium cells. After installation, repeater electronics andbatteries are encapsulated in a suitable material such as STYCAST, madeby Emerson & Cuming, Billerica, Mass., and sealed off by the contactplate 430 and epoxy sealant. Since there are many more chambers 450 thanrequired for a complete repeater, multiple repeaters can be accommodatedand/or backup batteries may be included in spare chambers. Since therepeater housing 410 isolates the electronics and battery from thedownhole pressure, its collapse pressure must exceed the maximumdownhole pressure. Considering the cavities as isolated tubes, adiameter-to-thickness (D/t) ratio of (10 mm cavity diameter)/(1.5 mmminimum wall thickness) is chosen. These values determine a maximallyallowable I.D. of about 3.15 inches. The collapse pressure Pc is givenby Pc=2 Ym (((D/t)−1)/(D/t)²)˜30,000 psi, where the material yieldstrength Ym is taken to be 120,000 psi. All chambers 450 are sealed byindividual seals and by Viton gasket rings that also serve as electricalinsulation against the contact rings 430. Electrically, the repeaterpressure housing 470 is a single, insulated electrode and the contactrings 430 form a second, namely a ground, electrode. The repeater worksby sensing radiofrequency voltages between these two electrodes,processing the modulated information, and sending out in response packetmessages modulated onto a radiofrequency carrier and applied betweenhousing and ground.

FIG. 18 illustrates the electrical characteristics of the system asinstalled in a pipe joint. The “hot” plates of the pin electrodes 370couple electrically (as opposed to magnetically) through the dielectriccoating 400 to the drilling fluid 500 and from there to the insulatedinner surface of repeater housing 410. The electric circuit is completedthrough the ground connection from the repeater's contact ring 430 tothe pin face 375 (not shown) and further through capacitive groundcoupling to the “cold” plates of the pin electrodes 370. Similarly, theinsulated outer surface of the repeater housing 410 couples electricallyto the “hot” side of the box electrode system 350, which communicatesthrough the transmission line(s) 360 with the pin electrode system 370on the far side of the same pipe joint. The return ground connection isprovided through the contact ring 430, the pin/box threads andcapacitive ground coupling to the “cold” plate. The pin electrode 370 inturn couples to yet another repeater (not shown), completing thecommunication path between adjacent repeaters. Due to symmetry, eachrepeater communicates with its two closest neighbors. Under favorabletransmission conditions such as highly dielectric non-conductivedrilling fluid, signals can travel even between repeaters that are morethan one pipe joint separated from each other. All electrode systemsinfluenced by a transmitting repeater are in resonance. All repeaterscontained in the repeater housing couple to all electrode segments,thereby providing full redundancy. Also important is the fact that allcouplings are weak in the sense that neither a short circuit nor an opencircuit within one segment can cause the remaining segments tomalfunction. However, such a failure is evident through a reduction inrepeater-to-repeater signal level. In the case of two electrode segmentsper electrode system, the loss of one segment causes a drop of −6 dB inreceived signal strength, sufficient to trigger a warning reportingcondition.

It has been found experimentally that the arrangement shown provides onaverage received signal strength (RSS) of about −60 dB below thetransmitter level, using oil as signal transmission medium at anoperating frequency of 27 MHz. The RSS drops by 10 dB for the case ofair as the medium and increases by about 25 dB for the case of fresh(i.e., poorly conductive water). As salinity and temperature of thewater increases, its conductivity, together with unavoidable leakagecurrents to the pipe metal, decreases the RSS. In the limit ofsalt-saturated brine, the received signal is about −75 dB below thetransmitter level. In the worst-case of salt-saturated brine and apartial electrode failure, the signal level through the remainingsegment is about −80 dB below transmitter level. Therefore, at a nominaltransmitter power of 10 dBm (0.7 V), the worst-case received signal isabout −70 dBm or 70 microvolts.

FIG. 65 is a screen shot from the CRT of an HP 3585A spectrum analyzershowing received signals through a prototype of the data transmissionsystem. The horizontal axis is frequency with a span of 100 kHz andcentered on 27.125 MHz. The vertical axis is received signal amplitudein dBm with −25 dBm at the top of the range and −125 dBm at the bottom.A radiofrequency signal with a frequency of 27.125 MHz is injected witha level of 0 dBm (0.2236 V) at the approximate location of repeater andthe detected signal at the approximate location of the next repeater isshown on the screen. The stronger signal (upper trace) corresponds tothe case of water as dielectric medium within the bore; the weakersignal (lower trace) corresponds to an empty, air-filled bore. Theapproximate ratio of signal strengths is about 32 dB or a factor of 40,which compares well to the expected ratio of dielectric constantsbetween fresh water (˜50) and air (˜1) at 27 MHz.

FIG. 19 further details the modifications required to convert a standardAPI rotary connection to data transmission capability. The box end 301has been enlarged radially and also in depth in accordance with FIG. 16and Table 16, pp. 24-25, in: “Specification for Rotary Drill StemElements—API Specification 7,” 40^(th) Edition, November 2001, AmericanPetroleum Institute, API Publishing Services, Washington, D.C. Thisspecification was designed as stress-relief measure for the box portionof a rotary tool joint. As a side effect, adequate space for therepeater housing 410 is created, provided that the tolerances on pinlength and box depth are held tighter than required by the APIspecification. Preferably, the machining operation is performed on newtool joints before the tool joints are welded to the tubulars. However,the backboring operation may also be performed on complete pipe joints,both new or previously used. For tool joints of typical dimensions, thespace available between the pin face 375 and the back of the box afterbackboring is about 2 inches (L_(CB)). The I.D. of a backbored box isapproximately 4.2 inches (D_(CB)), while the flow channel has a typicaldiameter of 3 inches, leaving a wall thickness available for the signalrepeater housings of approximately 0.6 inches (15 mm). The notation inFIG. 19 corresponds to the notation used in API Spec 7. It is importantfor the function of the data transmission system that the length of thepin (parameter “L_(Pin)”) is well defined and subject to much tightertolerances than defined by the API Spec 7. Therefore, machining the pinsection 303 includes checking for proper pin length and, if necessary,cutting to the proper length and re-surfacing the pin 320.

FIG. 20 shows a modified rotary connection with data transmissionelements 350, 360 and 370 installed and the epoxy coating 400 applied.The coating 400 not only serves as corrosion inhibitor, but also asprotection for the electrodes 350 and 370 and transmission line elements360. The coating 400 also serves as electrical insulator between theelectrodes 350 and 370 and the pipe metal as well as between theelectrodes 350, 370 and the drilling fluid. Typically, the coating 400will not achieve 100% coverage, which is not necessary for thefunctioning of the data transmission system. It is important, however,that the signal loss due to shunt currents flowing from the signal pathto ground is limited. From electrical modeling it has been found that ashunt current corresponding to a 1-mm wide gap, exposing bare metal overthe entire circumference in the pin/box interface to a fluid with 50 S/mconductivity is acceptable. Note that the pin face 375 is not epoxycovered. This surface sees rough handling on the rig floor and would notretain the coating. Instead, this metal surface serves as groundconnection to the repeater housing 410 once the rotary connection isfully tightened.

FIG. 21 a shows the components of FIG. 20 with the signal repeatersinstalled in the box. The repeater housing 410 occupies the straightpart of the box cavity, while the taper ring 420 fills the space at theback wall that tapers from the larger cavity (diameter “D_(CB)”) to thesmaller I.D. of the box tool joint (diameter “d”). The inner diametersof both the repeater housing 410 and the taper ring 420 areepoxy-coated. The surface facing the pin, i.e., the flat face of thecontact plate 430, remains bare metal. A snap ring (not shown) may beused to secure the repeater housing 410 in the box. The repeaters remainin a very-low-power state as long as the connection is not made up. Inthis state, the repeaters periodically power up for a short amount oftime, determine the capacitance of the insulated housing against groundand re-enter the very-low-power state if that capacitance is below athreshold, indicating that the contact plate 430 of the repeater housing410 is not in contact with a ground connection.

FIG. 21 b is the rotary connection as in FIG. 21 a, but in the made-up(tightened) state. The pin length had been dimensioned such that the pinface 375 contacts the repeater housing 410 while the connection is madeup and exerts compressional force on the repeater housing 410 and thetaper ring 420, keeping both components captive. This stopping actionalso prevents the connection from being overtorqued. In order to aidthis operation, the taper ring 420 can be configured as a spring elementand may be made from a suitable material such as beryllium-copper. Sincethe taper ring 420 is not essential for electrical connections, it mayalso be made from an elastomeric material. A secondary function of thetaper ring 420 is to protect the transmission lines from mechanicaldamage, particularly at the points where the transmission wire 360 hasto cross from the enlarged portion of the box into the tapered area andfrom the tapered area into the bore. The taper ring 420 is designed tocover both sensitive areas such as these areas are no longer exposed todrilling fluids and erosion does not take place as a result.

As soon as the electrical contact between the contact plate 430 and thepin face 375 is established, the capacitive load against ground as seenby the repeaters increases to its operating value. In response, therepeaters exit their dormant state and enter another low-power state inwhich the repeater's receivers are activated from time to time andlisten to transmission signals from active repeaters. Galvanic contactbetween the contact plate 430 and the pin nose 320 is not required. Aslong as these two surfaces are in close proximity with capacitivecoupling of a few 100 pF, the capacitive load against ground is withinits nominal operating range.

FIGS. 22 a and 22 b show the same rotary connection as FIGS. 21 a and 21b with the optional sensor insert 390 included. The two main parts arethe sensor insert 390 proper and a plug 395 that seals the pipe interiorfrom the recess machined into the box section 301 to house the sensorinsert 390. The plug 395 is made from a high-performance compositematerial such as polyetheretherketone (“PEEK”) or from a ceramic and ispermanently mounted in the box, while the sensor insert 390 isremovable. Such PEEK plugs can be custom made by Green, Tweed ofHouston, Tex. The sensor insert 390 is a fully sealed, self-containedpackage that screws into a form-fit machined recess in the tong sectionof the box end 301. The sensor insert 390 senses environmentalconditions in the outer borehole annulus or in the formation andcommunicates via capacitive coupling using radiofrequency signals withthe repeaters located inside the box. This communications channelconsist of a capacitive signal electrode 391 within the sensor insert390 that couples to a relay electrode 392 within the plug 395, which inturn couples electrically to the insulated repeater housing 410. Theelectric circuit is completed from the sensor insert stem 393 to thepipe metal to the rotary thread and to the contact plate 430 on therepeater. Repeaters and sensors have a master-slave relationship, inwhich a repeater interrogates the state of the sensor using speciallyformatted communication packets transmitted at radio frequency (e.g., at27 MHz) and the sensor responds with a status communications packet.

FIG. 23 enumerates some of the sensors that can be deployed in thesensor insert 390: Accelerometers to assess drillstring vibrations anddrilling dynamics; temperature sensors to map the temperature profilealong the borehole (and thereby detecting influx of connate fluids thatare hotter than the circulating drill fluids); pressure sensors to mapthe pressure gradient along the borehole, which is beneficial inbalancing borehole pressure against formation pressure; geophones to actas receivers in VSP (vertical seismic profiling) applications; electriccurrent sensors to map the conductive environment around the borehole;and others known to those skilled in the art.

FIG. 24 shows an electrical circuit block diagram of the variouscomponents inside the repeater housing 410. In this example, tworepeaters (A and B) are utilized. As shown, each primary cell has anintegrated fuse and diode, allowing cells to be shunted in parallel forredundancy and added ampere-hour capacity. Repeaters installed in drillcollar joints may require two battery cells as shown in FIG. 24. Thesupply voltage from the cell(s) feeds the core circuit 505 contained incartridge 460 that performs the function of a packet radio. Thecircuit's transmit/receive stage contains a tunable tank circuit 550,which is connected to (a) the repeater housing main body 470, and (b) tothe contact rings 430 on each end of the repeater housing throughferrules 465. Each core circuit 505 in cartridge 460 has a temperaturesensor, which allows for tracking the change in permittivity, and henceresonant frequency, of the electrodes systems.

A particular advantage of using two primary cells per repeater in thearrangement shown in FIG. 24 is the possibility to combine cells withdifferent discharge characteristics. While the first cell may be acommercial cell such as the aforementioned size ½ AAA lithium cell10-25-150 from Electrochem, the second cell can be a custom design withfavorable discharge characteristics. The commercial cells, to be ofuniversal use, must have a certain minimum current delivery capacity,typically in the milliampere range, which implies a minimum membranecross section, which in turn puts a floor under the self-dischargecurrent flowing at high operating temperatures. A custom cell design canbe “programmed” by means of limiting the membrane cross section for verylow current delivery (a few 100 microamperes), but with very favorableself-discharge characteristics. By combining such different cells,short-term, high-current demands can be met by the commercial cell,while the long-term power needs are met by the custom cell. In theextreme, the commercial cell may have long been depleted, while therepeater still functions based on the custom cell alone, albeit at areduced duty cycle and at data rates matched to the lower poweravailable.

FIG. 25 is a more detailed block diagram of core circuit 505. Thefunctions of a packet radio are realized by a microcomputer unit (MPU)510, a transmitter section 520, a receiver section 530, atransmit/receive (T/R) switch 540 and a digitally tunable tank circuit550. The MPU 510 controls the power supplied to all analog frontends aswell as to all sensors and clock circuits. Together with its own standbycapabilities, the MPU 510 can reduce the circuit's power consumption toa few microamperes during idle times. This functionality is important toachieve service times of up to and exceeding 1,000 hours. Also indicatedis the temperature sensor 560 that allows the MPU 510 to track thechange in dielectric permittivity exhibited by the electrodes.

The T/R switch 540 routes a transmit signal to the tank circuit 550 andthe electrodes; connects the tank circuit 550 and electrodes to thereceiver input; and also gates a low-amplitude transmit signal to thetank circuit 550 while receiving the reflected signal. A periodicalfrequency sweep across the range of permitted operating frequenciesallows the MPU 510 to determine the real and imaginary loads at theelectrode. The variable load conditions are compensated by switching inand out additional tuning capacitors contained in the tank circuit 550,allowing the MPU 510 to keep the resonance frequency and therefore theoptimum operating frequency in a narrow range.

The T/R switch 540 implements its functions under control of the MPU510. First, during a signal transmission interval, the T/R switch 540connects the transmitter section 520 to the tank circuit 550, butprotects the sensitive receive section 530 from the outgoinglarge-amplitude signal. During signal reception, the T/R switch 540connects the receive section 530 to the tank circuit 550, and enablingsignal reception by disconnecting the transmit section 520 from the fromthe tank circuit 550. Lastly, during idle periods, the T/R switch 540completely disconnects the tank circuit 550 from all other circuitry,presenting only a high-impedance load to the outside. The MPU 510 alsocommands the T/R switch 540 to disconnect from the outside world in theevents of low battery power or a detected hardware or software fault. Insuch a case the MPU 510 proceeds to shut itself down. Without power fromthe battery and/or a specific “transmit” or “receive” command from theMPU 510, the T/R switch 540 by default isolates the internal circuitryfrom the outside world. These features of the T/R circuitry 540 enablemultiple repeaters to operate in parallel using the same electrodes. Italso allows repeaters to continue to operate in case one or more of theparallel repeaters has ceased to operate normally.

The ability to re-tune the antenna tank circuit digitally and to adoptthe operating frequency to the operating conditions is particularlyadvantageous if the data transmission system is deployed in variable anda priori unknown drilling fluid systems. It has been foundexperimentally that a system tuned to 27 MHz for non-conductive,dielectric drilling fluids such as air, foam, oil or very fresh waterwill operate best in a frequency range around 20 MHz when operated inconductive drilling fluids such as brine. The reason is that the fluidincreases the ground capacitances of the pin and box electrodes, as wellas the ground capacitance of the repeater housing 410. Theaforementioned frequency sweep detects the electrical environmentcreated by the drilling fluid and allows the repeater MPUs 510 to switchto the operating frequency appropriate for the drilling fluid. It hasbeen found that the MPUs 510 need only to distinguish between twooperating bands: one for conductive fluids (˜20 MHz) and one fordielectric fluids (˜27 MHz), which makes certain that all repeatersagree on the same frequency band.

A special case may exist when new pipe is being added to a drill stringin the borehole. The situation is illustrated in FIG. 54. The pipestring in the borehole is typically filled with borehole fluid, whilethe new pipe joint may be empty. The repeaters in the new pipe arelistening in time periods “R” for the arrival of “beacon” signals thatwould allow them to synchronize to the existing repeater network. Sinceit is not possible for the “orphaned” repeaters to know the establishedoperating frequency band a priori, they listen for beacon signalsalternatingly on both bands by digitally adding capacitance to theantenna tank circuit to switch from around 27 MHz down to around 20 MHz.The frequency of the first beacon signal detected locks in the operatingfrequency band to use. Therefore, as the network builds up by addingmore repeaters, all repeaters automatically communicate on the samefrequency.

FIG. 26 is a block diagram of the optional sensor insert 390. Itcontains its own battery 950, processing unit (MPU) 910 and packet radiostages 920-950. Input data from the environment are conditioned,digitized and processed within the circuit using sensing, conditioning,and A/D conversion elements 960, 970, and 980, respectively. The circuitcommunicates with the associated repeaters by means of capacitivecoupling. This local communication may use the same carrier frequency asthe telemetry service, in which case time multiplexing is employed, ormay be allocated a separate frequency in a frequency-multiplexingmanner.

FIGS. 66-69 show an implementation that is suitable fordouble-shouldered rotary connections. As opposed to thesingle-shouldered, API-standardized design discussed up to this point,in double-shouldered connections, the back wall of the box is flat andmakes contact with the pin face 375 when the connection is fully madeup. Thus, a secondary shoulder is formed that takes up some of themechanical load and that prevents over-torquing the connection.According to the present invention, standard API connections may also bemodified as shown in FIGS. 66-69, thus improving the mechanicalcharacteristics of the connection and creating the “look-and-feel” of adouble-shouldered connection.

FIG. 66 shows cross-sections through the box and the pin after the boxhas been back bored to create a cavity suitable for a signal repeaterhousing. The new box backwall is flat and the length of the cavityequals the height of a double-shoulder repeater (1.5 to 2 inches [38 to50 mm]). A key seat 398 and/or a stress-relieve feature 399 may bemilled into the box.

FIG. 67 shows the box and pin cross sections after the transmissionelements box electrodes 350, transmission lines 360 and pin electrodes370 have been installed and embedded in coating 400.

FIG. 68 shows the box and pin cross sections after the double-shoulderrepeater housing 411 has been installed. A pin 397 on the repeaterhousing 411 engages with the key seat 398 in the box back wall, lockingthe repeater in place and preventing its rotation within the box cavity.The double-shoulder repeater 411 shares the same general constructionprinciple with API-type repeater 410; however, the repeater housing bodyand the contact rings 430 of repeater housing 411 are electricallyconnected to each other. Signal transmission and reception occursthrough coated surface electrodes 450 that communicate with the pinelectrodes 370 and the box electrodes 350, respectively, through theborehole fluid. The construction of the repeater electrodes 450 isgenerally the same as that for pin and box electrodes; e.g. the repeaterelectrodes are similarly embedded in epoxy coating 400.

FIG. 69 shows the cross section in the fully made-up position. The pinface 375 contacts the repeater and exerts a compressional force on therepeater that is transmitted to the box backwall. The pin face 375 alsoelectrically grounds the contact ring 430 and the repeater housing 411itself. Thus, the electrodes 450 become electrically “hot” and can actas radiofrequency transmitters and/or radiofrequency receivers.

FIG. 70 is a numerical simulation result generated by COMSOL (COMSOLMultiphysics, version 3.5a, COMSOL Inc. Burlington, Mass. 01803). Theplot shows the geometry inside the fully made-up rotary connection withthe pin in contact with the repeater housing 411. The streamline plothas been generated for the case of a dielectric drilling fluid such asoil-based mud. Shown are the field lines of the electric polarization,or equivalently, the field lines of the displacement current. In thisexample about 1% of the displacement current emanating from thetransmitting electrode, which in this case is a repeater electrode,arrives at the destination electrode, which in this case is the pinelectrode 370.

The repeater electrodes 450 may cover the entire inner annularcircumference (360°). In this case, the antennas of the repeaterelectrodes 450 couple azimuthally uniformly into the box electrodes 350and the pin electrodes 370 and the particular azimuthal orientation ofbox electrodes 350, pin electrodes 370 and repeater 411 and therefore ofrepeater electrodes 450 is of no particular consequence. In this casethe key 397 and the key seat 398 are optional.

The repeater electrodes 450 may also be split into two or morecircumferential segments as shown for the case of two segments perelectrode in FIG. 71. The number of segments should equal the number ofsegments in the box and pin electrodes 370 (see FIG. 11 a). In the caseof two segments per any such electrode, the repeater electrodes 450should be rotated by 90° with respect to the pin electrode segments 370and/or with respect to the box electrode segments 350. In order toachieve correct alignment, both pin and box electrodes must be mountedin fixed relationships with respect to the pin and box thread forms,respectively, such that in the fully made-up state corresponding pin andbox electrode segments either face each other or are rotated withrespect to each other by a fixed 90° angle. The angular orientation ofthe repeater electrodes 450 is fixed by the positions of the pin 397 andthe key seat 398.

In the case of azimuthally segmented antennas 450 as shown in FIG. 71,each segment is electrically connected to its own repeater within therepeater housing 411. The electrical connection is realized by routing astrand of each electrode 450 to the inside of the repeater housing 411and connecting the “hot” side of the electrode to the T/R switch 540 or940 and the tuning circuit 550 and 950. Due to the fixed, relativeazimuthal orientations of the electrode segments, each repeater weaklycouples to all electrode segments within the pin and the box. Any singlefailure of a pin segment, a box segment, a repeater electrode segment ora repeater can be tolerated and does not lead to a failure of the systemitself.

Also shown in FIG. 71 is a “dielectric window” 415 located in the sidewall of repeater housing 411. This window is realized by embedding acapacitive plate electrode inside a PEEK plug in analogy to theconstruction shown in FIGS. 22 a and 22 b. The window 415 is the mirrorimage to the plug 395 installed in the connection box. Both plugs or“windows” 395 and 415 act in series to allow the repeater electronics tocommunicate via radiofrequency with the optional sensor insert 390.

It is desirable to maintain the mechanical strength of adouble-shouldered rotary connection even when no data communicationfunctions are needed. In this case it is economical to replace therepeater housing 411 by a passive insert 414, shown in FIG. 72. Thepassive insert 414 has the same outside dimensions as a repeater housing411 and the two parts are interchangeable. As the name suggests, thepassive insert does not contain electronics nor does it carryelectrodes.

The conversion process to telemetry-enabled pipe for double-shoulderedrotary connections as illustrated in FIGS. 66-69 is also applicable tostandard API-type pipe. In this case, the conversion has the addedbenefit of improving the mechanical strength of the rotary connection.In order to maintain the strength improvement, either a repeater housing411 or a passive insert 414 must be installed before the connection ismade up.

FIG. 27 shows two connected pipe joints 30 in semi-transparent view tohighlight the data transmission system elements contained in the rotaryconnection. The make-up process of the rotary joint electrically couplesthe ground (“cold”) components of all electrodes and transmission linesthrough the pipe metal. The repeaters 410 or 411 are grounded throughcontact with the pin 320 of the adjacent tool joint. In connected pipejoints, the pin electrodes 370 and the repeater housings 410 or 411 arein close proximity, i.e. within a few centimeters, and are coupledthrough displacement currents (in dielectric fluids such as air, foam,very fresh water, oil, or water-in-oil emulsions) or through acombination of dielectric and galvanic currents in conductive fluidssuch as water, brine, or oil-in-water emulsions.

FIGS. 28 a and 28 b show schematically an exploded view of two pipejoint stacks with data transmission elements installed, but withoutsensor inserts. In FIG. 28 a the transmission lines 360, i.e. theinstalled wires, are mostly straight connections, while in FIG. 28 b thetransmission lines 360 form corkscrew-shaped spirals on the interiorborewall. Preferably, an entire assembly consisting of pin and boxelectrode systems and connecting wires is pre-assembled, tested andinstalled as a single unit in a pipe joint. Since pipe joints can be ofdifferent lengths, the straight-wire version requires each assembly tobe custom-made for a particular joint, whereas in the spiral-wireassembly length variations can compensated for by simply adjusting thepitch of the spiral. For clarity, the following figures show only thestraight-wire version, but it should be understood that the spiraledversion is equally applicable.

FIGS. 29 a and 29 b show schematically an exploded view of two pipejoint stacks with data transmission elements as well as sensorsinstalled. In FIG. 29 a, the sensor inserts 390 are housed in the boxesof standard-length pipe joints as described above, while in FIG. 29 bthe sensor inserts 390 are housed in special sensor subs 395, which areessentially short pipe joints, complete with rotary connections andinstalled data transmission elements 350, 360, 370, 380 and 410/411. Theadvantage of FIG. 29 a is a constant-length drill string with or withoutsensors; while the advantage of FIG. 29 b is the unchangedtorque/compression/tension load limit of the box sections. The sensorsub connection boxes have thicker walls compared to pipe joint boxes tocompensate for the loss in strength due to the insert recess and thedielectric window.

FIG. 30 shows some detail of the transmission system at the surface. Inparticular, an example of a table drive system using a kelly 70 isdiagrammed. The components, shown in exploded, see-through view, fromtop to bottom are the communications sub 700, the kelly 70, a lowerkelly valve 710 (shown as an example how valves are instrumented) and akelly saver sub 720. The communications sub 700 translates the signalstraveling on the data transmission system to signals compatible withexisting surface equipment. Since the communications sub is part of arotating drill string, the communications sub 700 is preferentiallybattery-powered and exchanges signals with the surface equipment 705 viaa radio link. Rig safety considerations, however, may demand wired-onlycommunications, in which case the inner portion of the communicationssub 700 rotates with the drill string and the outer portion is fixedwith respect to the rig, allowing a cable to be run between the outerportion and rig equipment. Signal transfer between rotating andnon-rotating parts is accomplished by a set of electrodes formingopposing plates of a capacitor in analogy to the components used in thedrilling jar (see below). Above the communications sub 700 may beanother valve or the swivel. The communications sub 700 interfaces tothe rest of the transmission system via a set of pin electrodes 370. Thekelly 70 is instrumented with the same transmission system components asa pipe joint, namely a repeater housing 410 or 411, box electrodes 350,transmission lines 360 and pin electrodes 370. Below the kelly 70 isshown an optional kelly valve 720, used to retain the mud level columnwithin the kelly 70 during make-up or break-out of rotary connectionbelow. The valve housing contains repeaters 410 or 411, box electrodes350 and pin electrodes 370. Instead of internal transmission lines,sections of miniature coaxial cables or simply individual wires 361 arerouted from the box electrodes to the outside of the valve body andtransition back into the valve body below the actual valve to connectwith the pin electrodes 370. The kelly saver sub 720 is an inexpensive,short piece of equipment used as wear item for frequent make-and-brakeoperations. It contains another set of repeaters 410 or 411, boxelectrodes 350 and pin electrodes 370, the latter connected by shorttransmission lines 360.

FIG. 31 is a schematic circuit block diagram of the communications sub700. The circuit employs the same base packet radio core circuit as therepeaters, including MPU 510, transmitter circuitry 520, receivercircuitry 530, T/R switch 540, and tuning circuit 550. Thecommunications sub 700 is powered by digital power supply 730 and analogpower supply 740. The communications sub 700 is further augmented byadditional processing and communication functions in communicationsblock 750, including a serial interface 751, message packetization andmessage queuing block 752, channel acquisition and channel release block753, session initiation and session control block 754 for communicatingwith the communications sub 700 via the MPU 510, as well as a two-wayradio 755, message dequeuing and message packetization block 756,network supervision block 757, and network recover block 758 thattogether enable communications with the outside world. These additionalfunctions handle the high-level end-to-end protocols including networksupervision functions and connectivity to the outside world such as arig data network via wireless links (as shown) or wired connections.Networking tasks performed in communications block 750 include, forexample, the handling of user data, which arrives in variable lengthmessages and its breakdown into fixed-length data packets; theallocation and the release of communication channels (channels will bediscussed below); the supervision of communication sessions plus avariety of general supervision and network maintenance functions. Inthis context, a “communication session” denotes the activities betweenthe time a continuous communication path between the downhole sub andthe surface sub has been established and the time when this path isbroken, for instance, when new pipe is added or a pipe joint is removedfrom the string. It is one of the tasks of the communication subs 700 togracefully recover from a broken-path condition and to quicklyre-establish communication as soon as the physical link has beenre-established. The communications link to the rest of the surfaceequipment is preferentially implemented as a wireless radio link.

FIG. 32 shows more detail of the data transmission system at thebottom-hole assembly (BHA). A crossover sub 540 translates between thedifferent thread forms used for pipe joints and for drill collars,respectively. The crossover sub 540 and the drill collars 40 areequipped for data transmission in similar fashion as the pipe joints.The interface sub 560 is a piece of instrumentation housed in a drillcollar 40 including signal-receiving and signal-transmitting ringelectrode 565 that translates the signals and protocols used by the datatransmission system to and from the signals and protocols used on theBHA instrumentation bus 568. The implementation of this bus isservice-company specific, but typically is a variant of MIL-STD-1553.Therefore, the interface electronics is partially generic and partiallyspecific to the equipment provided by the MWD/LWD/directional drillingservice company.

FIG. 33 is a simplified perspective view of a downhole interface sub560, seen from the top. Inside the box end of the rotary connection, thecontact ring 430 is visible as well as the signal-receiving andsignal-transmitting ring electrode 565, which has electricalcharacteristics similar to that of a repeater housing and is coatingwith epoxy coating 400. Therefore, the top end of a downhole interfacesub can transmit data to and receive data from the pin ends of collarjoints and/or crossover subs 540.

FIG. 34 is a schematic circuit block diagram of the electronics inside adownhole interface sub 560. A core circuit 800 similar to the repeatercircuit is employed to handle the radio packet functions and low-levelcommunication functions, augmented by additional circuitry 750responsible for high-level end-to-end communications, networksupervision functions and communication with the BHA instrumentationbus. The communications circuitry 750 is very similar to that in FIG. 31except that the two-way radio 755 is replaced by the interface 810 tothe BHA bus (pictured as an example MIL-STD-1553 bus), which is specificto each LWD/MWD vendor.

FIG. 35 is a schematic see-through view of an in-line instrument sub 570compatible with the data transmission system that can be deployed atvarious locations within a drill string. Such subs 570 may be used forgathering data along the drill string similar to the sensor inserts 390,with the additional functionality of being able, by virtue of beingpowered by battery packs with substantial capacities, to transmithigh-power stimuli, e.g. electric or acoustic signals that can bereceived along the drill string by sensor inserts, sensor subs or otherinstrument subs. In addition, such stimuli may travel betweenneighboring boreholes. In one possible implementation, as shown in FIG.35, the electronics resides in pockets 575 that are accessible from theoutside. The transceivers are located under windows 576 suitable to passsignals specific to the sensors to and from the borehole. Feed-throughsconnect the instrumentation pockets with the data transmission systemlocated on the inside. The data transmission system consists oftransmission lines 360 as previously described, terminating in a pinelectrode 370, and a box ring electrode 565 that is connected to theelectronics package 577 in the in-line instrument sub 570.

FIG. 36 is a schematic block diagram of the electronics inside aninstrument sub 570. The core circuit 1300 handles the low-levelcommunication functions and the packet-radio functionality. Theapplication-specific circuitry 815 manages the transceivers bygenerating stimuli signals using DAC 820, amplifier 830, and transmitter840 and processing received signals from external sources using sensor660, conditioning circuit 670, and ADC 680 to provide data to MPU 510for processing.

FIGS. 37 a and 37 b show the data transmission system in a drilling jar600. A jar 600 is often included in a BHA to aid in unlodging a stuckdrill string and/or stuck drill bit. The jar 600 is deployed in itsclosed position (FIG. 37 a). A stuck drill string stores large amountsof torsional and/or tensional energy, which is suddenly released whenthe jar is triggered and opens (FIG. 37 b). Then an internal hammerstrikes an internal anvil-like surface, and shock waves travel throughthe drill string. The signal transmission over this moving interface isaccomplished through semi-planar electrode systems 610 mounted on boththe stationary 601 and the moving parts 602 of the jar 600. As shown inFIG. 38, the layout of these electrodes 611 and 612 is very similar tothe circular electrodes in FIG. 11 a. The main difference is that thering electrodes of FIG. 11 a are laid out circumferentially, conformingto the connection's I.D., while the electrodes 611 and 612 of FIG. 38are laid out axially, conforming to the jar's I.D. Resonant tankcircuits are formed through the capacitance between the “hot” and the“cold” plates 611 and 612 and the integrated inductors. Since thestationary and moving “hot” plates 611 and 612 face each other throughthe drilling fluid 500 (see FIG. 39), the resonance tank circuits areweakly electrically coupled. If the electrode's lengths equal the traveldistance, e.g. 12 inches, and the electrode overlap equals ½ of thetravel distance, e.g. 6 inches, the coupling factor is the same for boththe “open” and the “closed” position. In this case, the signal strengthpassing through the jar is unaffected by the jar's open/closed status.As shown in FIG. 39, the electrode systems are located in shallowprotective grooves.

It is straightforward to generalize the scheme of moving electrodes torotational motion between moving parts and to a combination ofrotational and translational motions between such parts. Such designelements are useful for implementing brushless couplings below theswivel joint within the surface assembly, where a portion of the datatransmission system rotates with the drill string and another portion,which may be connected to the rig network over a communications cable,does not. Such wire-based network implementations are useful in rigenvironments that do not allow the use of radiofrequency signals forsafety reasons.

FIG. 40 shows a variation of the repeater circuit of FIG. 24 employingrechargeable batteries. The obvious advantage of such an arrangement isthat the batteries can be fully sealed within the repeater housing 410and repeaters can be used many times over. The necessary outsideconnections are provided through the contact rings 430 at each end ofthe repeater housing 410. As shown in FIG. 40, the contact rings 430 areelectrically connected through blocking capacitors 431, which allow thepassage of radiofrequency signals, but block the flow of d.c. current.Applying a d.c. voltage between contact rings 430 causes the electronicsto power up and the battery supervisory circuit to charge therechargeable battery cell(s). As shown in FIG. 41, many repeaterhousings 410 may be stacked together for charging purposes in a box,which is also used for storage and transport. Each stack of repeaters,which are serially connected to the charging station 620, alsointerfaces to a cylindrical antenna 432 that allows for operation,testing and programming of all repeaters during charging and/or intransit.

FIG. 42 is a schematical, exploded see-through of the top of a pipestring, represented by the topmost pipe joint 30—possibly suspended inthe slips—and disconnected from the surface communications sub. Ifnecessary, the checkout box 1000 as shown in FIG. 42 can be used tooperate the network without having the surface communications subconnected. The checkout box 1000 wirelessly communicates with theremovable portions of the communications system, i.e. the repeaters 410or 411. The checkout box 1000 emulates the functions of a surfacecommunications sub and can be operated from a local operator display andkeyboard 1010. A radio link for remote operation is also provided. Thischeckout box is useful, for instance, if a “soft” failure in a repeaterhas occurred and the affected pipe joint 30 needs to be replaced. The ID380 of the affected pipe joint is electronically retrieved by wirelesscommunication with the repeaters 410 or 411 inside the box end and isdisplayed on the local operator display 1010. As previously discussed,the pipe joint ID 380 is embossed on the rotary connection per APIguidelines, which allows the pipe joint to be identified and removedfrom the string. The checkout box 1000 has a slightly elongated pin thatalready contacts the repeater housing and hence activates the repeatersif the connection is only made up by hand.

FIG. 43 shows the general checkout procedure for a single pipe jointthat may be horizontally racked up. The “master” checkout box 1000operates in concert with the “slave” checkout box 1020 that is screwedonto the pin end of the pipe joint. These two boxes 1000 and 1020communicate over a radio link and can exercise all standard repeaterfunctions as well maintenance functions such as inquiries of usagehistory and temperature profile history. The boxes 1000 and 1020 have anelongated pin and a thicker contact plate, respectively, to allow properground contact through only hand-tightened connections.

A very fast way of checking out the functionality of repeaters, ofdownloading their internal memory, and for programming the internalmemory is a fully wireless connection. This mode is simplified if thedata transmission system operates in the radiofrequency range of 27 MHz.A narrow-band communication link tuned to a particular channel in the CBband at around 27 MHz can be established by a portable radio-typetransmission system and nearby located repeaters. In particular, anentire rack of pipe joints with data transmission system elementsinstalled may be interrogated very efficiently using the packet-radiocapabilities of the repeaters. In this situation, the peer-to-peercommunication protocol is replaced with a master-slave protocol, wherethe portable 27-MHz master system acts as master interrogating onerepeater at a time. Since the master system has a record of all pipejoint IDs, interrogating all repeaters within radiofrequency rangeshould yield a complete roll-call of stored pipe joint IDs. Anincomplete roll-call points to a data transmission system defect in thepipe joints embossed with those IDs.

FIG. 44 illustrates the principle of synchronizing repeaters. The taskis to transfer pipe maintenance data and pipe history data from depletedrepeaters to be removed from a pipe joint to fresh repeaters to beinstalled in the pipe joint. The new repeater housing 412 is stacked ontop of the old repeater housing 413, allowing all repeaters to exchangeradiofrequency signals through over-the-air coupling. Under direction ofthe master checkout box 1000, the old repeaters download storedmaintenance data to the new repeaters. Such data comprises pipe jointID, manufacturing records, service records, usage records, temperatureand pressure profiles (the latter requiring a pressure sensor insert),etc. After the download is complete, the old repeater housing 413 isremoved and recycled and the new repeater housing 412 is installed.

By using a combination of synchronizing repeaters, fast repeaterinterrogation using a wireless connection, and/or temporarily storingpipe information in data bases that may be remotely located, apermanent, continuous and traceable data record for each pipe joint canbe established. Such data records are desirable to trace the history ofeach pipe joint in order to maintain proper service schedules, predictremaining service lifetime or to assist in the analysis of failures. Inthe past such systems could not be made to work because the necessaryelectronics located within a pipe joint would not survive the rigors ofdownhole use and thermal pipe cleaning over the long run. The problem issolved by storing the pipe manufacturing, usage history and maintenancerecords in a combination of generations of replaceable repeaters andoff-line storage. Repeaters can be easily removed while, for example apipe joint is thermally cleansed at temperatures electronic componentscannot easily withstand.

FIG. 45 shows the general electrical block diagram of a checkout box1000 or 1020. Again, a version of the core circuit 1030 is employed asbasic building block, together with circuitry 750 implementing allhigher-level networking functions as well as the user and externalcommunication interfaces, including wired network interface 1035. The“master” box 1000 has pin electrodes 370 and a local display/keyboardinterface 1010; the “slave” box 1020 has a ring electrode 565 and nolocal interface.

FIG. 46 shows an example of a simple modulation pattern used to transmitdata between repeaters, i.e. between pipe joints. The example modulationis on-off keying (OOK), combined with Manchester modulation. A logical“0” is represented by approximately ½ of the bit time BT with theradiofrequency carrier turned on, followed by ½ of the bit time withouttransmission. A logical “1” is represented by ½ BT “off”, followed by ½“on”. Therefore, one bit is transmitted in one BT and a byte in 8 BT's.The bit time BT can be as short as about 1 microsecond or can madearbitrarily long, with a practical limit of about 1 millisecond.Measurements have shown that the data transmission system, when tuned to27 MHz, has a bandwidth of about 2 MHz and therefore can easily supportbit times as short as about 1 microsecond. More elaborate modulationtechniques such as QAM (quadrature amplitude modulation) can be employedto transmit more information during the same time frames. OOK, however,results in particularly simple receiver analog front ends that can beimplemented using power-saving techniques and simple envelope detection.The modulation scheme is also robust in the sense that it does notrequire communicating repeaters to operate on precisely the samefrequencies. This feature is useful since although each repeatercourse-tunes its tank circuitry to bring the resonance frequency intothe allowed operating range, timing drifts, aging effects andtemperature variations introduce repeater-to-repeater differences inoperating frequencies.

FIG. 47 elaborates on the structure of message packets that aretransmitted in single bursts between repeaters. A data packet as shownin FIG. 47 a includes a preamble, which is a constant bit pattern usedto synchronize transmitter and receiver, various control bits, theactual user data (payload) and cyclic-redundancy check (CRC) bits. FIG.47 b shows a so-called beacon packet, which does not transmit user data,but is used to establish a connection between repeaters for subsequentdata transfer.

FIG. 48 illustrates the packetization process of user payload data. Theuser data is formatted into variable-length messages, which in turn arebroken down into fixed-length payload chunks, each wrapped into a packetframe. The packets are transmitted using a modified time-division,multiple-access (TDMA) protocol. There are multiple channels, numbered 0and up, that can be requisitioned or released on demand, depending onthe user transmission rate. These channels access the physicaltransmission medium in a round-robin schedule, with each channel givenin this example access for 3 ms per time slot. Three milliseconds thenbecomes the minimum time a packet resides at a particular repeater,including reception time, transmission time and confirmation time.

The example given in FIG. 48 assumes the parallel transmission of alonger message in Channel 0 and a shorter message in Channel 1. Inaddition, Channel 2 is in the process of being requisitioned, acondition that is signaled by a series of beacon packets placed in thetime slots of Channel 2. Channels 3 and onward are dormant in thisexample as no activity takes place in these time slots. The message inChannel 0 is broken down into a start packet, a continuation packet andan end packet. The message in Channel 1 requires only a start packet andan end packet. All messages are required to begin with a start packetplaced at the synchronization point of the channel the message is placedin. These synchronization points occur typically once every second andare staggered by channel as shown in FIG. 48. If a repeater is notsynchronized to a particular channel, it follows the trail of beaconsignals transmitted in that channel until it finds the synchronizationframe, denoted by “*” in FIG. 48. With the number of channels selectedas 6 and a per-channel slot time of 3 ms, the round-robin cycle takes 18ms. The overall repetition time of 1 second can accommodate 330 packets,split among 6 channels for 55 packets/channel/repetition time. Themaximum message length is then given by 55× number of bits per packet.The minimum message length is a single, “start/end” data packet.

The messaging protocol has been organized around variable messagelengths and fixed synchronization points to minimize the powerconsumption in the repeater's electronics. Once synchronized, a repeatercan predict when the next synchronization point will occur, respectivelywhen the start packet of the next message will arrive. As soon as theend packet for a message has been processed, no more data packets willarrive on that particular channel until the next synchronization pointand the repeater can, assuming no more channels are active, enter alow-power state with only an internal timer counting down to the nextsynchronization point. In addition, the repeater electronics enters lowpower states whenever a gap in communication is to be expected, such asduring time slot reserved for idle channels. The concept of channels isalso useful when the processing and transmission workload is distributedamong multiple repeaters. In short, channels will be allocated toavailable repeaters in an equitable manner. This allocation can bedynamically and transparently modified as necessary. These concepts willbe further discussed below. The discussion of the following figures ispurposefully simplified as it assumes only one repeater per pipe joint.

Another advantage of the messaging protocol is the automatic timesynchronization between repeaters. Each repeater maintains local time towake up from low-power states to receive and/or transmit data packets attheir allocated times. The clocks maintaining local time are subject todrifts, in particular during low-power intervals and at elevatedtemperatures. Even without data transmission requests from the terminalpoints, the system generates “heartbeat” transmissions every 1 second,events to which every repeater resynchronizes its internal timekeeping.In addition, since the retention time of a packet at a node is known andthe total number of transmissions a packet has undergone is known aswell, each repeater can calculate “absolute time” by inspecting theheader information in a packet, i.e. it can synchronize itself to thetime maintained at the surface and/or in the BHA. Assuming a worst-casedrift of 100 ppm, the maximum time drift during a 1-second interval is0.1 ms. Reception and retransmission of a packet is subject to randomprocessing time jitter at every node; however, that jitter tends toaverage out over many nodes, leaving only a constant delay per node plusclock “noise” that is proportional to the square root of the number ofnodes in series. From these considerations it can be estimated that aBHA periodically adjusting its clock to the time information from thearriving heartbeat packets will stay synchronized to the surface (rig)clock to within 0.1 ms. In comparison, an autonomous BHA clock,synchronized once at the surface and free running for 100 downhole hourswould have to maintain time accurate to an impossible-to-achieveprecision of 3×10⁻¹⁰ to track surface time to within 0.1 ms.

FIG. 49 illustrates the transport of a single data packet through thenetwork of connected pipe joints. Time increments of 1 millisecond areassumed for this example. Five connections are pictured (left) and thestates of the repeaters are symbolized as “T” (transmitting), “R”(receiving), and “ ” (idle) on the right-hand side of the diagram. Thesequence begins with the uplink of a packet from the lowest repeater toits next neighbor. Since the physical level transmissions arebidirectional, any uplinked packet automatically bounces back to theprevious repeater. The previous repeater maintains a copy of a sentpacket and compares is to the bounced-back version it receives in thenext time slot. Therefore, every bit in every packet is always checkedfor transmission errors. Assuming no transmission errors occur, thepacket proceeds through the chain of repeaters at a speed of onerepeater hop per time increment or 10 meters per millisecond. Thus, atypical uplink propagation speed would be 10 km/s, ignoring extra packetprocessing time required for error detection and correction.

FIG. 50 diagrams the events caused by an intermittent transmissionproblem. It is assumed that the transmission between the second andthird repeater (from bottom) is temporarily corrupted (“X”). Thereceiving repeater detects the problem from the CRC checksum. Asingle-bit error can be typically corrected on the fly and the repeaterforwards a corrected version. In this example, serious corruption of thedata packet is assumed, in which case the repeater skips transmissionand instead prepares itself for another reception event. The previousrepeater does not see packet confirmation, waits for a pre-specifiedtime interval and retransmits the stored packet (“re-uplink”). If thisrepeat transmission is successful, the uplink continues, otherwise theretransmission attempts continue for a number of times.

FIG. 51 shows the transport of multiple data packets at full uplinkcapacity. Data packets percolate serially from bottom to top at a speedof 10 km/s and at a rate of 1 packet every 3 milliseconds. Assuming apayload capacity of 500 bit/packet, the net (user) data rate in thiscase is 166 kbps. The end-to-end user data transfer in this case isunidirectional.

FIG. 52 shows the events in case of a transmission problem in afull-capacity uplink condition. Again, it is assumed that a singlefailed uplink event (“X”) triggers a re-uplink that succeeds andrestarts the pipeline. The repeater below and once-removed from thefailure location sees the same packet bounced twice back to it. This isthe signal that a transmission error has occurred somewhere above andinstructs the repeater to temporarily stall transmissions. This patternof repeated bounces percolates down the pipeline and causes allrepeaters to stall for one cycle. In effect, the memory of the entirepipeline is used to buffer the data in transit until the problem hascleared. The stalled repeater retransmits the previous packet (althoughit already had been confirmed in an earlier cycle). If the problem hasbeen cleared in the meantime, the stalled repeater will receiveconfirmation for the just-repeated packet; if not, it will receive acopy of an older packet. In the latter case, the repeater will stall foranother cycle and will keep re-sending the packet it holds in memory. Apipeline stall condition routinely occurs when a new connection is madeat the surface. In that case, the surface communications sub istemporarily disconnected from the drill string, which causes all uplinkin progress to stall. There is little use in repeaters endlesslyattempting to uplink data that becomes increasingly stale. Instead, therepeaters give up after several repeated uplink attempts, clear theirinternal data buffers and enter a low-power mode in which they transmitonly beacons in order to re-establish communications from scratch.

FIG. 53 introduces full bidirectional data transfer. The totaltransmission capacity has been split 50:50 between uplink and downlinkcapacity, resulting in uplink/downlink rates of one packet per 6milliseconds. This mode is asymmetric with respect to transmission speedand latency: uplink occurs at a rate of 1 hop per millisecond, equal to10 km/s, but the downlink requires 5 milliseconds per hop, equal to 2km/s. Obviously, by simply turning the diagram on its head, the downlinkcould be made the faster direction, with the uplink being the slowerdirection. The downlink transfer handles both user data such as commandsto downhole equipment, but also plays a role in end-to-end networkcontrol functions between the surface communications sub and thedownhole interface sub. Such network control functions are data rateadjustments, switches between bi- and unidirectional modes and thehandshaking procedure after the physical connection between surface suband downhole sub has been established.

FIG. 54 shows a pipe string in the process of new pipe being added atthe surface. The downhole interface sub keeps the network synchronizedby sending test data packets through the pipeline. These test packetsstall at the orphaned repeater, which sends out beacon signals in anattempt to attract new repeaters to the network. The beacons are shortpackets without user data, but with timing information necessary tosynchronize with the TDMA time slots. In the meantime, the stallcondition percolates downwards and reaches the downhole interface subthat keeps track of the stall situation. As soon as a new pipe joint hasbeen added, the new repeater picks up the beacons emitted by theorphaned repeater, synchronizes itself to the pattern and acknowledgesthe beacon by a short transmission to the orphaned repeater thatswitches to regular transmissions. The link-up event is also recognizedby the surface communications sub, which in response attempts anend-to-end communication with the downhole sub. If successful, the twosubs handshake by exchanging control packets, exchange information aboutthe bandwidth required by the user data streams in each direction,further exchange information about network status and jointly determinemode and transfer capacity for the next data transmission period. Themode chosen may range from the basic MWD mode in which the pipeline isconfigured for bidirectional data transfer and a single packet isuplinked every second, to SWD (seismic-while-drilling) mode, in whichlarge amounts of data is uplinked in unidirectional mode at high datarates.

FIG. 55 shows an implementation of a “swarming” protocol, useful forinterrogating a complete network status and/or for collectingdistributed sensor data. The surface system sends out a special packetrequesting a swarm, which causes each repeater to respond with astatus/data message and also to forward another swarm request packet. Asthe request packet travels downhole, a “swarm” of response packetstravels uphole with the net result that the surface receives a stream ofstatus messages, where each message corresponds to the status of onenetwork node.

The forgoing discussion has been purposefully simplified by assumingonly one repeater per pipe joint. In reality, there are 2-3 repeatersoperating in parallel per pipe joint to implement a fully redundantnetwork. It is advantageous to evenly distribute the transport workloadacross all repeaters that are available. That way, the failure of asingle repeater becomes immediately known throughout the network andcorrective action can be taken. This capability is implemented using thechannel concept. As already mentioned, a channel is a logicalcommunications link between downhole and surface that exists independentof other links. The primary (highest-priority) uplink connection isChannel 0 and the primary (highest-priority) downlink connection isChannel 1. The number of channels is arbitrary; however, the mostpractical number is the redundancy parameter m (i.e., the number ofrepeaters per repeater housing) times the number of directions, i.e. 2.If, for example, m=3, then a good number of channels is 6, numbered 0(highest priority) to 5 (lowest priority). In the example of full-speedunidirectional uplink, the three repeaters take turns handlingconsecutive packets. The control bits contained in each packet indicatethe channel number, which in turn directs the packet to its managingrepeater and also indicates priority and up/down direction. By examininga single data packet, a repeater can tell which channel the data packetbelongs to. Since channels follow a strict round-robin time schedule,this information is sufficient to synchronize any repeater to thechannel timing. Further, every repeater checks the incoming channelnumber against the repeater's table of channels to determine whether ornot it should handle the packet (error-checking and forwarding) or not.These tables are initialized with preferred values that allocate to eachrepeater an equal share of the workload; however, the tables can bedynamically adjusted. Operational redundancy and failure tolerance areachieved by any repeater's ability to listen-in to channels that aremanaged by other repeaters. Since channels are independent logicalconnections, it is possible to have connected channels and brokenchannels at the same time. Repeaters are programmed to accept beaconsignals received during a listen-in period—which indicate a brokenchannel—and to service that channel if the broken condition persistsover a pre-determined time. This latency time differs between repeatersbased on their initial channel allocations and current workloads,thereby avoiding a race condition in channel pick-up.

FIG. 56 shows the sequence of events during partial failure ofrepeaters. FIG. 56, top, is the normal state in an m=3 repeater group.Channels 0 (uplink) and 1 (downlink) are allocated to Repeater A,Channels 2 and 3 are routed through Repeater B and Channels 4 and 5 gothrough Repeater C. In FIG. 56, center, a sudden, total failure inRepeater A is assumed, an event that breaks Channels 0 and 1. Therefore,a “broken pipe” condition exists in Channels 0 and 1, while Channels 2-5continue to operate. After the number of attempted repeat packettransmissions has been exhausted, the last working repeater or repeaterstransmitting on Channels 0 or 1 begin to send out beacons in an attemptto find working repeaters. The chain of beacons is intercepted byRepeaters B and C during their listen-in periods. Repeater B is the nextin line for service and responds to the beacon exactly like a freshrepeater in a newly-added pipe joint would. Repeater B adds Channel 0 toits table of channels and starts regular service on Channel 0, inaddition to Channels 2 and 3. The computation of channel pick-up timetakes into account the length of the channel list, resulting in RepeaterC to react first to the beacons that are still sent out on Channel 1.So, in return, Repeater C picks up Channel 1 and acknowledges the startof service, resulting in the new configuration shown in FIG. 56, center.The first packet transmitted through the channel that has been picked upis a network service packet that describes what has happened in terms ofwhich repeater has picked up which channel, including the serial numberof the pipe joint the event has occurred in. This packet is processed bythe communications sub or interface sub at the receiving end, logged andis reflected back to the other end of the network. At the surface, thehardware failure is logged, formatted as an operator alert andtransmitted over the rig network. Although the breakdown of a singlerepeater is a “soft” failure that does not immediately requireintervention, the affected pipe joint is tagged for removal and serviceat the next opportunity. The sequence of events described above repeatsitself in case another failure on the same group of repeaters occurs. Asshown in FIG. 56, bottom, assuming Repeater B has broken down as well,Repeater C goes through the same motions by picking up all brokenchannels in beacon mode and starts managing the entire workloadconsisting of Channels 0-5.

The doubled or tripled workload per repeater results in faster depletionof the working repeater(s)' batteries. The repeater's MPU continuouslymonitors battery status by measuring battery voltage under load and alsoestimates remaining battery life based on the history of workload andtemperature profiles over past times. If the MPU's battery maintenancealgorithm indicates that the battery charge becomes critical, loadshedding occurs. This process is shown in FIG. 57 using the sametriple-repeater example shown in FIG. 56. The tripled workload,accumulated over time, has stressed Repeater C to the point wheresuccessful completion of the current downhole run at full data rate maynot achievable (FIG. 57, top). Repeater C examines its channel table inan attempt to reduce the workload. Starting at the highest-numbered,lowest-priority channels, Repeater C sends out a maintenance packetindicating the impending shutdown of a certain channel, which itidentifies by channel number. The communication interfaces immediatelyrespond by taking that channel off-line, i.e. stop schedulingtransmissions on that channel. Repeater C proceeds to purge the channelfrom the channel list and will no longer respond to packets that occurin that channel's time slot. Channel shedding continues until a morestable operating point is found (FIG. 57, center)). A severely stressedrepeater will service only Channels 0 (uplink) and 1 (downlink) as shownin FIG. 57, bottom. The capability of channel shedding enables repeatersto throttle the data throughput and therefore manage their batterycurrent drain to a certain amount. Since the communication subs arefully aware that a stressed repeater is shedding capacity, they willfurther assist that repeater by throttling the data rates at theirrespective ends and reducing the per-channel throughput based onpriority information supplied with the user data.

FIG. 58 is a simplified flow diagram of a repeater's operation meant toillustrate the aforementioned operational concepts. The repeater'sfirmware program 1100 starts from power-up entering the low-powerDormant State 1102. While dormant, the repeater checks every few secondsfor a connection-made-up condition by measuring the resonance frequencyof the electrode tank circuit 550. If the resonance frequency indicatesthat no ground connection is present, the Dormant State 1102 continues.In the presence of a ground connection, indicating that a pin thread iscontacting the repeater housing, the program starts (“spawns”) theChannel Process at 1104. A single channel process can serve a singletransmission channel and therefore typically runs in multiple copies onthe same repeater. The Channel Process 1106 first enters the low-powerInactive State 1108. While inactive, the Channel Process 1106 checks forbeacon signals by periodically turning on the repeater's receiver.Without a beacon received, the Inactive State 1108 continues. It isimportant to note that a data packet cannot trigger an exit from theInactive State 1108; only a beacon can. If a valid beacon is detectedduring the time frame allocated for the specific channel is received,the beacon is answered by sending back an acknowledgement signal, thechannel is marked as active and the currently running channel processspawns another copy of itself. That copy starts monitoring the nextchannel as per channel table. In the meantime, the original channelprocess enters the Active State 1110. While active, the channel processoperates the channel by watching for data packets, verifying theirintegrity, correcting bit errors if possible and forwarding correctpackets. If a packet is corrupted beyond repair, the channel processdoes not forward it, which is equivalent to not sending a confirmationback to the originating repeater. In this case the current channelprocess expects the originating repeater to re-send a copy of the datapacket several times and attempts to decode those copies as they arebeing received. Likewise, a forwarded packet requires confirmation fromthe next repeater. The channel process maintains a copy of the packetbeing forwarded and repeats its transmission should no confirmation beensent back. This loop continues for about 10-20 retries, after which thecurrent data packet is discarded. The channel process proceeds to sendout a string of beacon signals to signal a persistent problem with thechannel. If another repeater in the next pipe joint is available to takeon this channel, it will do so at this point. Otherwise, the string ofbeacons is terminated after a time-out period, an error packet is sentback through the network using one of the remaining channels and thechannel is closed by terminating the current channel process. The onlychannels that cannot be closed in this fashion are the basic channels 0for uplink and 1 for downlink. While active, the channel process is alsoresponsible for assessing the remaining battery lifetime and the overallhealth of the repeater's hardware at 1112. If the repeater's history,the battery's voltage, the temperature profile and the current workloadindicate that the repeater may not live through the current deploymentcycle, it will start shedding channels (except 0 and 1) by notifying thenetwork and by shutting down the associated channel processes.

The data transmission system is designed for up to 2,000 downholeservice hours or one year, whatever comes first, before the batteries inthe repeater housings need to be replaced. 2,000 service hourscorrespond approximately to 100 drilling days or to 50,000 feet ofdrilled hole. Of those 50,000 feet, it is assumed that 40,000 feet arenot in reservoirs, require a data rate of 100 bps, and 10,000 feet arein reservoir zones, requiring a data rate of 1,000 bps. Thus, the totalnumber of bits transmitted during a service interval is 100 bps×3600sec/hr×1600 hrs+1000 bps×3600 sec/hr×400 hrs=2 gigabit. The number ofdata packets processed is 40 million, if one data packet carries about50 payload bits. Further, it is assumed that two repeaters per pipejoint (m=2) are used and equitable workload distribution is providedsuch that the total number of data packets processed per repeater is 20million. The current consumption of a repeater strongly depends on itsactivity level. With the analog frontends turned off and the MPU insleep mode, the electronics power consumption is close to the batteryself-discharge rate. In Active Mode and during intervals when the MPU isprocessing data packets, the MPU consumes an estimated 5 mA. Transmitterand receiver, when turned on, consume an additional 5 mA each. Each datapacket requires reception, transmission and confirmation for a total of3 ms. Therefore, the electric charge consumed by processing a datapacket is about 30 microcoulomb. The electric charge consumed during theentire service interval is 30 microcoulomb/packet×20 million packets=600coulombs or 0.17 Ah. In addition, non-productive time is necessary toestablish and to monitor communications by transmitting and receivingbeacon signals. The bulk of this non-productive time occurs when pipejoints are assembled in stands—i.e. with connection made—and therepeaters listen for beacon signals in their inactive states. Thisactivity costs 10 mA for 1 ms every second or a total charge of 0.01 Ahover a maximum of 1,000 hours. Beaconing is a power-intensive activityfor a short time period, but occurs only very infrequently on the levelof an individual repeater. Its contribution to the energy budget can beestimated as another 0.01 Ah. The repeaters also require quiescentcurrent for their periodic power-up, whether or not they are in service.This current drain is about 10 mA for 1 millisecond every 10 seconds,equivalent to a constant 1 microampere. Over the course of one year,this drain accumulates to a charge of 0.01 Ah.

The Electrochem 10-25-150 lithium cell has a rated capacity of 0.5 Ah ata discharge current of 2 mA. The cell loses about 0.1 Ah in capacitywhen discharged with 2 mA for 200 hours at 150° C. This loss isequivalent to an extra current drain of 500 microampere. Below about100° C., the self-discharge current is relatively small; between 100° C.and 150° C., and a linear relationship between temperature and dischargecurrent is assumed.

FIG. 59 shows an example well layout used to calculate a worst-casetemperature profile. Given a typical earth temperature gradient of 1°C./100 ft. and a typical well profile of 10,000 ft. vertical, 2,000 ft.build-out and 3,000 ft. horizontal, a simplified temperature profile forthe bottom repeater of 20 days below 100° C. and 10 days at 150° C. canbe assumed. The battery charge lost due to self-discharge is thenapproximately 0.12 Ah or about 25% of the rated capacity. The batterycharge expended on actual work is 0.19 Ah/3=0.06 Ah, leaving a remainderof 0.32 Ah in the battery. It can be further assumed that on the nextwell the pipe joints will be shuffled such that the repeaters havingexperienced worst-case bottom-hole conditions will remain in the upperportion of the well and at temperatures below 100° C. This“low”-temperature deployment cycle will reduce the battery charge byanother 0.06 Ah to 0.26 Ah, which is sufficient for one morehigh-temperature cycle costing 0.18 Ah. These assumptions are not validfor repeaters installed in collars, since the collars as part of the BHAexperience high temperatures on every well. The collar repeaterbatteries lose 25% of their nominal charge per well and need to bereplaced on every other well under nominal operating conditions: twodeployments consume, including self-discharge, 0.36 Ah or 72% of thebattery's capacity.

The preceding discussion focused on possible failures in the repeaters.Equally important are failures in the transmission lines. As alreadydiscussed, the electrical coupling between repeaters and the electrodesis weak with the advantage that a failed transmission segment is onlyseen as an incremental load against ground. The particular failure, i.e.an open circuit or a short against ground, and the location of thefailure along the electrodes and/or transmission lines determine thenature of load, which can be capacitive or inductive. From therepeater's point of view, a broken transmission segment and an operatingtransmission segment in parallel manifest themselves as a drop inreceived signal strength (RSS) of approximately 6 dB, i.e. one-half ofthe original signal strength. In the case of doubly-redundanttransmission elements, the RSS drops to ⅔ (−3.6 dB) for a singlehardware failure and to ⅓ (−10 dB) for a double hardware failure. Asshown earlier, the worst case of a highly conductive medium and a shortto ground result in a receiver voltage about 30 times stronger than thenoise r.m.s. voltage. By taking the absolute minimum RSS as three timesthe noise voltage, it is found that the system operates at or above a 20dB signal-to-noise margin. Thus, a loss of 66% in RSS still allows thesystem to function. As an added safeguard, each pair of repeatersexperiencing low RSS may elect to communicate at a reduced data rate. Asan example, sending each bit not just once, but in 10 copies allows thereceiving repeater to average over 10 bit intervals and to recover anadditional 10 dB in signal-to-noise margin, i.e. the amount lost in adouble failure. This process is a simple example of signal recoverythrough digital filtering with the benefit that the receiver filtersrealized in hardware do not need to be changed.

It is desirable to immediately detect and diagnose not only repeaterfailures but also transmission line failures. For this reason, everyrepeater calculates a received signal strength indicator (RSSI), whichis stored on-board all repeaters as part of the history log. The twoRSSI values for the uplink direction and for the downlink direction arealso part of the status messages that are collected periodically fromall repeaters by the surface communications sub. A failed transmissionline segment manifests itself by two repeaters separated by a singlepipe joint length that show lower RSSI levels for downlink and foruplink, respectively, compared to the RSSI levels pointing away from thefailed line segment. This pattern immediately points to the pipe jointcontaining the failed transmission line segment. The pattern of a failedtransmission line is best recognized by the surface interface thatperiodically sends a “swarm” request down the communications pipeline.As discussed above, such a global status request traveling down the pipetriggers a status response from each repeater, resulting in a swarm ofstatus packets arriving at the surface. This maintenance procedure isrepeated every 1-10 minutes. The surface system analyzes the totality ofall status responses, flags all “soft” and “hard” failures and presentsa unified system health status to the operator. The system health statusincludes a list of pipe joints that should be replaced at the nextopportunity and an estimate of remaining operating time based on historyand current usage. If the estimated remaining time is too low forsuccessful completion of the current job, the surface system takespreventive action by requesting reduction in data throughput from thedownhole interface.

A further advantage of the data transmission system is the automaticdetection and calculation of drill string length. In current rigoperations, the number of pipe joints added to the drill string is oftenrecorded manually, leading to omissions and therefore errors incalculations of total depth and true depth. In the system describedabove, every repeater has in memory the actual pipe joint length as partof the maintenance and repair records. These values can be easilyrecalled during operation and added up to calculate the current, actualdrill pipe length.

FIG. 60 shows a drill string instrumented with the data transmissionsystem and the optional sensor inserts 390, located in pipe joints 30and/or sensor subs 395. A typical drill string of 15,000 ft. lengthcontains about 500 pipe joints or collar joints, which translates to 500measurement points distributed along the drill string. In the example ofFIG. 60, the sensors are geophones listening to the noise emanating fromthe drill bit 60 during drilling. These sounds travel through the rockand are partially reflected at boundaries where the acoustic impedancechanges. Such acoustic targets may be indicative of hydrocarbon bearingzones, rock beds or cavities. The data gathered by the geophonescorrespond to multiple acoustics paths through the formation and can beused to solve by inversion for an acoustic impedance map around theborehole, thus illuminating portions of the underground formations notpenetrated by the borehole.

FIG. 61 illustrates a similar principle using the drill string asdistributed sensor array, but in this case a dedicated seismic surfacesource 1200 is used instead of the bit noise. The advantage are theknown power density function of the transmitter and the ability togather both reflective and transitive transmission properties as theacoustic wave travels through the rock formations. Similar techniqueshave long been used in wireline logging and are known as VerticalSeismic Profiling (VSP). The key difference between VSP and the presentinvention is the ability to conduct seismic surveys at any time duringbrief intervals between drilling activity, e.g. every time another drilljoint is added. In contrast, VSP is a survey that runs on wireline aftera well has been drilled. VSP uses a limited number of wireline-mountedgeophones, requiring repetitive operations at different stations in theborehole. In contrast, the present invention gathers seismic data alonglarge sections of the borehole at fine resolutions, using a minimum ofor no additional rig time.

FIG. 62 shows a drill string instrumented with geophones 390—possiblylocated in sensor subs/inserts 395—and with seismic sources housed ininstrument subs 570. The cartoons FIG. 62 a and FIG. 62 b illustratedifferent phases in the firing sequence of the seismic sources. Thesources are fired sequentially with all geophones 390 listening to eachseismic event. The received forwarded and reflected waves are indicativeof the acoustic impedance map of the surrounding rock. This map can beapproximately calculated by inverting the received seismic waveforms, aprocess that may lead to the identification of hydrocarbon accumulationsin the vicinity of the borehole.

FIG. 63 shows the situation of two adjacent boreholes, a situationuseful for increasing the drainage rate of a reservoir or forgravity-assisted drainage. Both wellbores are instrumented similar tothe single wellbore of FIG. 62. In addition to receiving signals fromsources located in the same wellbore as the receivers, the acousticwaves transverse the rock space between the wellbores and are receivedin the adjacent well. FIGS. 63 a and 63 b picture two phases in theserial firing sequence of the seismic sources. This technique, known ascross-well tomography greatly benefits from the large number of sensors(1,000 or more in this case) made available by the data transmissionsystem.

For all seismic examples shown in FIGS. 60-63 it is essential that eachlocal geophone 390 can determine the arrival of seismic waveforms interms of “absolute” time, i.e. the delay after a seismic surface sourceor a source located in an instrument sub or the BHA has been fired. Asdiscussed earlier, time information is distributed at least once everysecond throughout the data transmission system. Although it takes thetime information up to a second or more to travel through the datatransmission system, each node can reconstruct absolute time from thenumber of transmissions the heartbeat packet has undergone. Overall, theentire data transmission system and therefore every repeater and everysensor insert or sensor sub remain synchronized to known or “absolute”time to within 0.1 ms. Given a representative speed of sound in aformation of 5,000 m/s, the time uncertainty translates to a distanceuncertainty for seismic mapping of 0.5 m.

Using the data transmission system as a high-density distributedreceiver system as described in the foregoing discussion represents astep-change over previous attempts to distribute sensors along a drillstring, the main difference being that instead of receivers spacedseveral thousand feet apart as discussed in the literature, the receiverarray as part of the present data transmission system consists ofhundreds of nodes that are only 30 ft. (10 m) apart from their nextneighbors. The resulting high-resolution sampling allows for preciseback projection of the received time-domain data into the rock spaceadjacent to a borehole or between boreholes with spatial resolutions of,e.g. bed boundary locations, improved by 1-2 orders of magnitude.

It should be understood that this invention is not limited to theparticular embodiments disclosed, but it is intended to covermodifications within the spirit and scope of the present invention asdefined by the appended claims. All such aspects of the invention areintended to be covered by the appended claims.

I claim:
 1. A downhole signal transmission system for providingcommunications along a drill string of downhole components comprising aplurality of interconnected downhole components, comprising: a pluralityof signal repeaters spaced along said drill string of downholecomponents, said signal repeaters being receptive to radiofrequencysignals having information modulated thereon; and at least one of saiddownhole components comprising: a first end containing a resonancecircuit, said first end interconnecting with an end of a first adjacentdownhole component, a second end, said second end interconnecting withan end of a second adjacent downhole component, and a conductor throughthe downhole component, wherein said radiofrequency signals havinginformation modulated thereon are coupled from said first adjacentdownhole component to said downhole component through said resonancecircuit; and are transmitted through said conductor of said downholecomponent to at least one of said repeaters.
 2. A downhole signaltransmission system as in claim 1, wherein said resonance circuit iscoated by an insulative coating and wherein said radiofrequency signalsare coupled from said first adjacent downhole component to said downholecomponent through said insulative coating.
 3. A downhole signaltransmission system as in claim 1, wherein said conductor is coated byan insulative coating.
 4. A downhole signal transmission system as inclaim 1, wherein said downhole component comprises one or more of thefollowing: pipe joints, pup joints, drill collars, heavyweight pipe,jars, kellys, subs, saver subs, crossover subs, instrumentation subs,sensor subs, interface subs, communication subs, drilling jars and/orcheckout boxes.
 5. A downhole signal transmission system as in claim 1,wherein the frequencies of said radiofrequency signals are in a rangefrom 1 MHz to 1 GHz.
 6. A downhole signal transmission system as inclaim 1, wherein said at least one downhole component comprises at leasttwo resonance circuits and said conductors, configured to provideredundant communication paths through said downhole component.
 7. Adownhole signal transmission system as in claim 1, whereby a failure ofone or more of said signal repeaters does not inhibit the transmissionof said radiofrequency signals through said drill string of downholecomponents.
 8. A downhole signal transmission system as in claim 7,wherein the number of said signal repeaters is substantially larger thanthe number of signal repeaters necessary to receive and transmit data.9. A downhole signal transmission system as in claim 1, furthercomprising a sensor that communicates with at least one of said signalrepeaters.
 10. A downhole signal transmission system as in claim 9,wherein said sensor comprises at least one of the following: anaccelerometer, a temperature sensor, a pressure sensor, a geophone, anacoustic sensor, and an electric current sensor.
 11. A downhole signaltransmission system as in claim 9, wherein said sensor investigatesproperties of a formation adjacent a borehole including the drill stringof downhole components.
 12. A downhole signal transmission system as inclaim 1, further comprising a sensor downhole component with at leastone sensor, said sensor downhole component being connected in said drillstring of downhole components so as to enable communications of sensorsignals along said drill string of downhole components using saiddownhole components and said plurality of signal repeaters.
 13. Adownhole signal transmission system as in claim 12, wherein said sensorcomprises at least one of the following: an accelerometer, a temperaturesensor, a pressure sensor, a geophone, an acoustic sensor, and anelectric current sensor.
 14. A downhole signal transmission system as inclaim 12, wherein said sensor investigates properties of a formationadjacent a borehole including the drill string of downhole components.15. A downhole signal transmission system as in claim 1, wherein saidplurality of interconnected downhole components have rotary connections,and wherein at least one of said rotary connections is adapted to acceptsaid at least one of said signal repeaters.
 16. A downhole signaltransmission system as in claim 1, wherein said radiofrequency signalsare modulated with said information using on-off keying.
 17. A downholesignal transmission system as in claim 1, wherein said information istransmitted in packets.
 18. A downhole signal transmission system as inclaim 1, wherein said information is transmitted in bursts.
 19. Adownhole signal transmission system as in claim 1, wherein saidradiofrequency signals are modulated with information identifying aparticular downhole component and information regarding said particulardownhole component is communicated along said drill string of downholecomponents using said downhole components and said plurality of signalrepeaters.
 20. A downhole signal transmission system as in claim 1,wherein said radiofrequency signals are modulated with informationidentifying a particular signal repeater and information regarding saidparticular signal repeater is communicated along said drill string ofdownhole components using said downhole components and said plurality ofsignal repeaters.
 21. A downhole signal transmission system as in claim9, wherein said radiofrequency signals are modulated with informationidentifying a particular sensor and information regarding saidparticular sensor is communicated along said drill string of downholecomponents using said downhole components and said plurality of signalrepeaters.
 22. A downhole signal transmission system as in claim 1,further comprising a sensor downhole component with at least one sensor,said sensor downhole component being connected in said drill string ofdownhole components so as to enable communications of sensor signalsalong said drill string of downhole components using said downholecomponents and said plurality of signal repeaters.
 23. A downhole signaltransmission system as in claim 1, further comprising an in-lineinstrument sub that is connected between respective downhole componentsin said drill string of downhole components and configured to gatherdata along a borehole and/or to transmit stimuli that can be receivedalong the drill string of downhole components by at least one sensor.24. A method of providing communications along a drill string ofdownhole components comprising a plurality of interconnected downholecomponents, comprising the steps of: generating radiofrequency signalshaving information modulated thereon; providing a plurality of signalrepeaters spaced along said drill string of downhole components, saidsignal repeaters being receptive to said radiofrequency signals;providing a downhole component having a first end containing a resonancecircuit, said first end interconnecting with an end of a first adjacentdownhole component, having a second end interconnecting with an end of asecond adjacent downhole component, and having a conductor through thedownhole component; coupling said radiofrequency signals from one ofsaid first and second adjacent downhole components to at least one ofsaid signal repeaters using resonance coupling; transmitting saidradiofrequency signals through said conductor of said downholecomponent; and coupling said radiofrequency signals to the other of saidfirst and second adjacent downhole components using resonance coupling.25. A method as in claim 24, further comprising overcoating saidresonance circuit of said downhole component with an insulative coating,wherein said resonance circuit is coated by an insulative coating andwherein said radiofrequency signals are coupled from said first adjacentdownhole component to said downhole component through said insulativecoating.
 26. A method as in claim 24, further comprising coating saidconductor with an insulative coating.
 27. A method as in claims 24,wherein the frequencies of said radiofrequency signals are in a rangefrom 1 MHz to 1 GHz.
 28. A method as in claims 24, further comprisingconfiguring at least two of said resonance circuits and said conductorsin said downhole component so as to provide redundant communicationpaths through said downhole component.
 29. A method as in claim 24,further comprising providing a plurality of signal repeaters whereby afailure of one or more of said signal repeaters does not inhibit thetransmission of said radiofrequency signals through said drill string ofdownhole components.
 30. A method as in claim 24, further comprisingproviding a sensor that communicates with at least one of said signalrepeaters, said sensor comprising at least one of the following: anaccelerometer, a temperature sensor, a pressure sensor, a geophone, anacoustic sensor, and an electric current sensor.
 31. A method as inclaim 30, wherein a sensor downhole component with said sensor isconnected in said drill string of downhole components so as to enablecommunications of sensor signals along said drill string of downholecomponents using said downhole components and said plurality of signalrepeaters.
 32. A method as in claim 24, wherein said plurality ofinterconnected downhole components have rotary connections, furthercomprising connecting at least one of said repeaters to a downholecomponent using at least one of said rotary connections.
 33. A method asin claim 24, further comprising modulating said radiofrequency signalswith said information using on-off keying.
 34. A method as in claim 24,further comprising transmitting said information in packets.
 35. Amethod as in claim 24, further comprising transmitting said informationin bursts.
 36. A method as in claim 24, further comprising modulatingsaid radiofrequency signals with information identifying a particulardownhole component and communicating information regarding saidparticular downhole component along said drill string of downholecomponents using said downhole components and said plurality of signalrepeaters.
 37. A method as in claim 24, further comprising modulatingsaid radiofrequency signals with information identifying a particularsignal repeater and communicating information regarding said particularsignal repeater along said drill string of downhole components usingsaid downhole components and said plurality of signal repeaters.
 38. Amethod as in claim 30, further comprising modulating said radiofrequencysignals with information identifying a particular sensor andcommunicating information regarding said particular sensor along saiddrill string of downhole components using said downhole components andsaid plurality of signal repeaters.
 39. A method as in claim 30, furthercomprising connecting an in-line instrument sub between respectivedownhole components in said drill string of downhole components andconfiguring said instrument sub to gather data along a borehole and/orto transmit stimuli that can be received along the drill string ofdownhole components by at least one sensor.
 40. A method as in claim 24,further comprising providing in the drill string of downhole componentsa sensor that investigates properties of a formation adjacent a boreholeincluding the drill string of downhole components.